Curv-Aluminium Fibre Metal Laminates-2

Optimal Bonding Conditions ofCurv-Aluminium Fibre Metal Laminates Stephen Handscombe u4402525 Supervised by Dr. Shankar Kalyanasundaram 30 th September 2011 A thesis submitted in part fulfilment of the degree of Bachelor of Engineering Department of Engineering Australian National University ll This thesis contains no material which has been accepted for the award of any other degree or diploma in any university. To the best of the author’s knowledge, it contains no material previously published or written by another person, except where due reference is made in the text. Stephen Handscombe 29 September 2011 © Stephen Handscombe lll Acknowledgements I would like to thank my supervisor Shankar Kalyanasundaram for his guidance and support throughout this project. His time and experience helped me get through some of the more difficult stages of the project and it has truly been a pleasure working with him. Your light- hearted approach boosted my spirits when in doubt! I would like to thank Anthony Sexton and Sudharshan Venkatesan for their continuous assistance and the time they sacrificed to help me along the way. Without your help I would not have been able to make my samples, work the Instron or setup the Aramis. Thank you to Dave Tychsen-Smith and Ben Nash for allowing me to use machinery in the engineering workshop and teaching me how to operate it. Thank you to Zbigniew Starchuski for the assistance you provided and the access you gave me to your materials lab. Finally I would like to thank my friends and family. Thank you for putting up with me over the last 8 months and for the encouragement you provided me along the way. Your support throughout my degree is greatly appreciated. lv Abstract This thesis studied the bonding of aluminium-polypropylene (Curv) fibre metal laminates. Previous work has studied the mechanical properties and stamp formability, but has not measured the strongest way to bond the aluminium and Curv. This paper investigated the bond strength created by different adhesive films and surface treatments by measuring stress and strain under loading. Double-lap shear samples were created and tensile loading was applied, with loading measurements taken and strain development recorded by image capture. The testing and analysis produced a sample type with a bond that withstood larger tensile loads than the polypropylene laminate. v Table of Contents Acknowledgements ................................................................................................................. iii Abstract ....................................................................................................................................iv List of Figures ..........................................................................................................................vi Chapter 1: Introduction........................................................................................................... 1 1.1 Background ....................................................................................................................... 1 1.2 Project Motivation............................................................................................................. 1 1.3 Aims and Contributions .................................................................................................... 2 Chapter 2: Literature Review.................................................................................................. 3 2.1 History of Fibre Metal Laminates..................................................................................... 3 2.2 Mechanical Testing of FMLs............................................................................................ 4 2.3 Aluminium-polypropylene FMLs ..................................................................................... 5 2.3.1 Curv ....................................................................................................................... 5 2.3.2 Surface Treatment and Adhesion........................................................................... 6 2.3.3 Thickness of Adhesive Layer .............................................................................. 10 Chapter 3: Method ................................................................................................................. 11 3.1 Construction.................................................................................................................... 11 3.1.1 Materials .............................................................................................................. 11 3.1.2 Surface Treatment................................................................................................ 11 3.1.3 Gluing Process ..................................................................................................... 12 3.1.4 Cutting Samples................................................................................................... 13 3.2 Testing............................................................................................................................. 13 3.2.1 Aramis and Instron Programming ....................................................................... 13 3.2.2 Microscopic Analysis .......................................................................................... 15 Chapter 4: Single Adhesive Layer Results ........................................................................... 17 4.1 Untreated Samples ..........................................................................................................17 4.1.1 Gluing Failure...................................................................................................... 17 4.1.2 Cutting Failure ..................................................................................................... 18 4.2 Instron Double-Lap Shear Tests...................................................................................... 19 4.2.1 Failure Type......................................................................................................... 19 4.2.2 Stress at Initial Failure ......................................................................................... 20 4.3 Surface Topography........................................................................................................ 22 4.3.1 Surface Conditions of Aluminium and Curv....................................................... 23 4.3.2 Xiro 23.110.......................................................................................................... 25 4.3.3 Xiro 23.601.......................................................................................................... 26 4.3.4 Gluco.................................................................................................................... 27 vl 4.4 Aramis Strain Analysis ................................................................................................... 28 4.5 Scope for Further Testing ............................................................................................... 32 Chapter 5: Multiple Adhesive Layer Results....................................................................... 34 5.1 Untreated Samples ..........................................................................................................34 5.2 Instron Double-Lap Shear Tests...................................................................................... 35 5.2.1 Failure Type......................................................................................................... 35 5.2.2 Stress at Initial Failure ......................................................................................... 36 5.3 Aramis Strain Analysis ................................................................................................... 38 5.4 Discussion ....................................................................................................................... 43 Chapter 6: Conclusions and Further Work......................................................................... 45 6.1 Conclusions..................................................................................................................... 45 6.2 Further Work................................................................................................................... 45 Appendix.................................................................................................................................. 47 A1 Statistical Analysis .......................................................................................................... 47 A2 Aramis Strain Evolution.................................................................................................. 48 Bibliography............................................................................................................................ 52 vll List of Figures Figure 2.1 - Load-Displacement curves of Curv specimens ...................................................... 5 Figure 2.2 – Ranking of Surface topographies in terms of peel energy .................................... 6 Figure 2.3 - Untreated aluminium 2024-T3 ............................................................................... 7 Figure 2.4 - Chromic acid anodised aluminium ........................................................................ 7 Figure 2.5 - Sulphuric chromic acid etched aluminium ............................................................. 8 Figure 2.6 – Chromic acid anodised surface topology (left), Phosphoric acid anodised surface topology (right) .......................................................................................................................... 8 Figure 2.7 - SEM photographs of fracture surface with 0% γ-APS concentration .................... 9 Figure 2.8 - SEM photographs of fracture surface with 3% γ-APS concentration .................... 9 Figure 2.9 – Influence of adhesive layer thickness (h a ) on experimental and theoretical fracture loads of epoxy/aluminium single lap joints ................................................................ 10 Figure 3.1 - PHI PW220C-X4A Gluing press with sample being prepared............................. 12 Figure 3.2 – Layering of materials in sample preparation........................................................ 13 Figure 3.3 – Specimen prepared for testing in the Instron........................................................ 14 Figure 3.4 – Stochastic paint and mask area of sample............................................................ 15 Figure 4.1 – A failed untreated aluminium specimen............................................................... 18 Figure 4.2 – Extension vs. loading graph of alkaline Xiro 23.110 sample 1............................ 19 Figure 4.3 – Failure methods for each sample set .................................................................... 20 Figure 4.4 – Stress at initial failure of every tested sample...................................................... 21 Figure 4.5 – Average stress at initial failure of each sample type with confidence intervals... 22 Figure 4.6 - Untreated aluminium, 5x resolution in left image, 50x in right image................. 23 Figure 4.7 - Etched aluminium, 5x resolution in left image, 50x in right image...................... 23 Figure 4.8 - Anodised aluminium, 5x resolution in left image, 50x in right image ................. 24 Figure 4.9 – Untreated aluminium Gluco sample..................................................................... 24 Figure 4.10 – Left image is 50x resolution of Curv surface. Right image is 50x resolution of Curv surface with Gluco........................................................................................................... 25 Figure 4.11 – Anodised Xiro 23.110 sample at 50x resolution................................................ 26 Figure 4.12 – Etched Xiro 23.110 sample, initial cohesive failure highlighted ....................... 26 Figure 4.13 – Xiro 23.601 sample 3 resolution 50x, white patches are adhesive film............. 27 Figure 4.14 – Aramis strain capture of an etched Xiro 23.601 sample .................................... 29 Figure 4.15 – Aramis strain capture of an etched Xiro 23.110 sample .................................... 30 Figure 4.16 – Aramis shear strain capture of an anodised Xiro 23.110 sample ....................... 31 Figure 4.17 – Aramis strain capture of an anodised Gluco sample .......................................... 32 Figure 4.18 – Parameters for tests of the adhesive film thickness............................................ 33 Figure 5.1 – Anodised 6 adhesive layer Gluco sample............................................................. 35 Figure 5.2 – Initial failure types for each sample set................................................................ 36 Figure 5.3 – Stress at initial failure, grouped into sample types............................................... 36 vlll Figure 5.4 – Average stress at initial failure of each sample type with confidence intervals... 37 Figure 5.5 – Comparison of confidence intervals of all Xiro 23.110 and Gluco tests ............. 38 Figure 5.6 – Normal strain on an anodised 6 adhesive layer Xiro 23.110 sample at initial failure........................................................................................................................................ 39 Figure 5.7 – Normal strain of an anodised Gluco 6 adhesive layer sample ............................. 40 Figure 5.8 – Normal strain of an anodised 3 adhesive layers Xiro 23.110 sample .................. 41 Figure 5.9 – Shear and normal strain for adhesive film thickness tests.................................... 42 Figure 5.10 – Samples with 6 layers of adhesive film.............................................................. 42 Figure 5.11 – Shear strain capture of a 6 adhesive layer sample.............................................. 43 Figure 5.12 – Multiple adhesive layer sample after testing in the Instron................................ 44 1 Chapter 1: Introduction 1.1 Background A Fibre Metal Laminate (FML) is a composite material made by layering sheets of high strength aluminium alloy with a fibre/epoxy composite. The development of FMLs began in the late 1970’s as one of a number of new materials to be utilised in the growing aerospace and high technology industries [1]. Exhibiting high stiffness-to-weight ratio, strong resistance to crack propagation and fatigue and high impact resistance, [1] FMLs have many advantages over existing alloys and metals. FMLs can be categorised into 3 groups: Aramid-Fibre- Reinforced Aluminium (ARALL), Glass-Fibre-Reinforced Aluminium (GLARE) and Carbon-Fibre-Reinforced Aluminium (CARAL) Laminates [2]. 1.2 Project Motivation To develop a comprehensive understanding of the properties and limitations of an FML, there must be an understanding of the mechanical properties of the metal, the fibre laminate and the bonding agent. There have been numerous studies conducted on the mechanical properties of FMLs. Past research has focused both the aluminium and the fibre laminates, and how different adhesive agents change the performance and mechanical properties of the FML [2][3][4]. Structures made out of FMLs have been designed considering ideal mechanical and bonding properties determined by the Classical Laminate Theory. This theory assumes perfect bonding conditions between aluminium and fibre laminate layers. In practice, achieving perfect bonding conditions is difficult, if not impossible. During the production process, numerous complications can lower the quality of the bond. A greater understanding of bonding limitations will be achieved through testing and analysis. The adhesion between the composite laminate and aluminium is vital to the overall performance of the FML. It is therefore imperative that the mechanical properties of the bonding agent are known. Understanding the strengths and the weaknesses of different bonding agents, adhesive thickness and surface treatments will provide a greater understanding of the limitations of FMLs. 2 1.3 Aims and Contributions The aim of this thesis is to determine the tensile strength and peeling strength of adhesive films used with different surface treatments in aluminium-polypropylene FMLs. Tensile tests followed by stress and strain analysis will measure bonding success. Previous work has investigated how material properties of FMLs are improved or diminished by different bonding processes, but has neglected to measure the mechanical properties of the adhesives used in the process. This research will address the following unique contributions to the field of FMLs: • Measure the tensile strength of Gluco and Xiro 23.601 and 23.110 adhesive films. • Test 3 different adhesive films and 3 different surface treatments to find the combination that yields the highest shear strength of an aluminium-polypropylene FML. • Test the effect the thickness of the adhesive film has on the strength of a bond. • Produce an aluminium-polypropylene FML where failure under loading occurs in the Curv laminate before the bond. 3 Chapter 2: Literature Review The aim of this chapter is to introduce the reader to the composite material industry; specifically fibre metal laminates. This chapter investigates the initial production of FMLs and the development of them over the last few decades. It will then focus on the key materials used in their production, looking closely at the superior material properties and limitations of each. The materials used in the FML from this thesis will be introduced. Finally, a look at previous work on aluminium surface treatments and adhesive layer thickness will conclude the literature review. 2.1 History of Fibre Metal Laminates Fibre Metal Laminates are a part of the rapidly growing sector of the composite material science industry. Development of FMLs began in the late 1970’s. Their adoption by the commercial sector over traditionally used metals and alloys is not widespread, with room for growth. For example, Airbus currently uses FMLs for fuselage and internal plane part construction [7]. FMLs can be categorised into 3 groups separated by the material used in the laminate layer: glass fibre reinforced (GLARE), carbon fibre reinforced (CARAL) and aramid fibre reinforced (ARALL). FMLs were first commercialised and patented in the 1980’s by AKZO, a Dutch chemical company [1]. Initially, FMLs were made with either a glass-fibre reinforced laminate or an aramid fibre reinforced laminate with an aluminium alloy. The aluminium layers were treated with an acid based anodising process to promote adhesion of the laminate. Commercialisation began in the late 1980’s, where FMLs were being used for aircraft fuselage, wing and internal floor construction. During initial research and the first years of commercialisation, ranges of tests were conducted on FMLs to compare their material properties to those of monolithic aluminium. A number of favourable properties of FMLs were found: • They were up to 20% lighter than aluminium. • Their high-speed impact properties were far superior to monolithic aluminium. • They had excellent burn-through resistance. • They had easy impact identification. Permanent damage could be found with visual inspection due to plastic deformation. • They had excellent resistance to crack growth, propagation and material fatigue. The superior material properties of FMLs were proven in a series of tests conducted by Airbus from 1990 – 1994 [1]. In these tests, glass-fibre reinforced aluminium was compared with common aluminium alloys then used in aircraft, lithium aluminium alloys and 6013 4 alloys. Barrels made of the FML and the aluminium alloys were constructed, and 100 000 fatigue tests were conducted on all barrels; at 1000 per week over approximately 4 years. The aim of the tests was to replicate the damage to the fuselage of an operational aircraft. Hammers, saws and milling machines were used to replicate dents, cracks and corrosion damage respectively. The FML barrel survived the testing, whilst the barrels made from other materials had to be repaired after significant crack growth. It was not until 2000 that work began on aluminium-polypropylene FMLs [27]. Recent study has indicated that aluminium-polypropylene laminates have exhibited excellent specific mechanical properties and superior impact performance to monolithic aluminium. Materials with the described properties have potential application in the automotive industry, where weight savings and impact damage reductions are important issues. Current work involves the testing of the stamp formability of aluminium-polypropylene FMLs, and will be discussed in greater detail in the following sections. 2.2 Mechanical Testing of FMLs To fully understand the limitations and potential applications of FMLs, extensive mechanical testing has been conducted under varying conditions and parameters. Such testing aimed to prove that FMLs exhibit superior mechanical properties to monolithic metals, their alloys and standalone fibre based laminates. Additionally, given the vast number of surface treatments, adhesive agents, materials and material grades, the study of different combinations for specific applications is vital. Earlier work has demonstrated that adhesive choice and bonding conditions have a significant effect on the tensile strength of FMLs. Poor interfacial adhesion resulted in a 10% reduction in the interlaminar shear strength in one body of work [3]. Furthermore, impact behaviour, one of the favourable properties of FMLs, was significantly influenced by the success of the interfacial bonding. Impact tests conducted under ASTM D7136 guidelines found that impact damage size is greater with poor interfacial adhesion. Good adhesion bonding showed better resistance to low velocity impacts with corresponding contact forces 25% higher than specimens with poor adhesive bonding [4]. With superior impact properties and a lower density than that of monolithic aluminium, FMLs would serve as a favourable replacement to the current metals and alloys used in automobile body panels. Automobile body panels are most commonly stamp formed [25]. Previous work has looked at the viability of stamp forming FMLs. Testing on aluminium-polypropylene FMLs was conducted comparing the surface strain of stamp formed samples to that of stamp formed monolithic aluminium. The aluminium-polypropylene laminate exhibited lower strain. Additionally, increasing the tool radii did not increase the strain in the aluminium- polypropylene laminate. In the monolithic aluminium samples, strain increased with increasing tool radii [11]. S 2.3 Aluminium-polypropylene FMLs 2.3.1 Curv Curv, a self-reinforced 100% polypropylene woven-fibre laminate, is a lightweight, impact resistant composite designed by Propex Incorporated, a subsidiary of British Petroleum. Curv has potential applications in the automotive industry due to its recyclability, low density (≈0.9g/cm 3 ), and impact resistance (impact strength increases as temperatures decrease). It is an excellent candidate for usage with aluminium in FML production, where reducing weight and increasing impact resistance are primary goals. The failure and fracture behaviour of Curv has been tested to determine the limitations of the material and its viability in the FML and automotive industries. It was found that Curv does not behave as an elastic material, due to somewhat poor consolidation during mechanical loading, resulting in multiple failure stages. Figure 2.1 highlights the poor consolidation of Curv during tensile loading. The fracture energy of Curv - 47.2/kJ/m 2 ) [12] - is more than 50% larger than that of unreinforced polypropylene, approximately 30/kJ/m 2 [16]. Figure 2.1 - Load-Displacement curves of Curv specimens [12]. 2.3.2 Surface Treatment and Adhesion The focus of this thesis will be research regarding the bonding of FMLs. Past work has shown that there exist three key factors that influence the success of the interlaminar bonding of FMLs; surface treatment, the adhesive film/agent used and the choice of curing process. Success in all three areas is vital to creating a strong bond. To create a successful bond, the adhesive layer must have some way in which it can adhere to the surface of both the aluminium and the laminate. Mechanical interlocking is the process 6 where an adhesive agent solidifies within the oxide layer on the surface of a substrate. The interlocking of an adhesive into the irregularities of the surface of the material is the key source of adhesion in a bond. The substrate needs to be sufficiently rough and irregular to allow the adhesive to interlock. There are two methods to create a surface suitable for an adhesive to mechanically interlock: mechanical roughening and chemical pre-treating. Past work has shown that mechanical roughening improves the joint strength, but not to a significant extent [18]. The introduction of an abrasive agent to roughen the surface (mechanical roughening) followed by cleaning with an ethanol-based liquid has been proven to increase the strength of interlaminar bonding between aluminium and polytetrafluroethylene [10]. Additionally, mechanical roughening methods (such as grit- blasting) do not produce desirable surface topographies for establishing mechanical interlocking with an adhesive. This can be attributed to the poor success of mechanical interlocking when roughness is produced on the macro-level (as with grit-blasting and other mechanical roughening techniques) [18]. Roughness produced on the micro-level has been shown to produce successful mechanical interlocking [17]. Chemical pre-treatments are an excellent method of roughening surfaces on the micro-level. Literature on measuring the peel energy of various surface topographies has shown that there are two types of surface topographies that promote the strongest mechanical interlocking. High-angle pyramid structure with dendrites and club-headed nodular structure produce peel energy strengths more than 3 times those of a flat smooth surface when bonding copper foil to an epoxy laminate, see Figure 2.2 for extensive rankings [17]. The other surface topography, high-angle pyramids with dendrites gives surfaces similar peel strength. Figure 2.2 – Ranking of Surface topographies in terms of peel energy [21]. 7 Chemical pre-treatments of the surface are more successful at improving bond strength. Sulphuric chromic acid etching is the most commonly used method of treating the surface of aluminium. Chromic acid anodising is another widely used method. Etching and anodising pre-treatments increase the thickness of the oxide layer on the surface of aluminium, creating a network of pores and whiskers (1-2µm thick, see Figures 2.3 - 2.5). These pores and whiskers are somewhat similar to one of the favourable surface topographies listed in Figure 2.2, the club-headed nodular structure. Figure 2.3 - Untreated aluminium 2024-T3 [2]. Figure 2.4 - Chromic acid anodised aluminium [2]. 8 Figure 2.5 - Sulphuric chromic acid etched aluminium [2]. Etching and anodising processes create a system in which mechanical interlocking can occur between the aluminium and an epoxy resin (the adhesive) [2]. Other work has shown that the anodising process is less susceptible to mechanical damage than the etching process [14]. This is because the anodising process is easier to control and therefore creates a more consistent surface topography than etching. Anodising and etching processes produce a surface finish approximately 20% rougher than that of untreated aluminium. Furthermore, both processes lead to higher surface energies than those of untreated aluminium 2024-T3 [2]. Anodising aluminium creates a deep, porous surface topography. Adhesive film applied to an anodised surface typically penetrates to the bottom of these pores [21]. A bond produced under theses conditions will have strength between that of the adhesive and that of the oxide layer. The etching process creates a similar surface topography, but with shallower pores. Figure 2.6 highlights the differences in the surface topographies of anodised and etched aluminium surface treatments. Figure 2.6 – Chromic acid anodised surface topology (left), Phosphoric acid anodised surface topology (right) [21]. 9 Along with surface treatments, primers can increase the adhesive strength of a metallic surface. Primers are typically used to promote adhesion on smooth surfaces that would not otherwise bond to adhesives. Previous work has investigated the success of primers at increasing the bond strength of untreated monolithic aluminium [9]. The introduction of a silane compound is a method of promoting adhesion between untreated aluminium and polypropylene. The use of primers to increase adhesion and environmental resistance of aluminium surfaces is a widely established practice. γ-aminopropyltriethoxy silane (γ-APS), when applied to the surface of untreated aluminium, greatly improves the lap-shear strength between aluminium and polypropylene. Maximum lap-shear strength was achieved at a γ- APS concentration of 3% [9]. Figures 1.7 and 1.8 show the fracture surface on the aluminium. The fibres in Figure 2.8 exhibited far greater plastic deformation than those in Figure 2.7. The fibres in Figure 2.8 fail parallel to the direction of force application, indicative of larger shear loadings before failure than those experienced by the sample in Figure 2.7. Figure 2.7 - SEM photographs of fracture surface with 0% γ-APS concentration [9]. Figure 2.8 - SEM photographs of fracture surface with 3% γ-APS concentration [9]. Parameters of the curing process directly affect the final strength of the bond between layers. Heating temperature, cooling rate, curing pressure and time are factors that influence the strength of the completed bond [10]. Porosity in the glue significantly reduces the interlaminar shear strength of FMLs. Mechanical testing showed a drop in interlaminar shear 10 strength of up to 30% when using samples with a porous adhesive layer due to failure of the curing process [13]. 2.3.3 Thickness of Adhesive Layer Compared to the body of work looking at the role of surface topography on successful adhesion, little has been done measuring the role of the thickness of the adhesive agent. It is reasonable to assume that the thickness of an adhesive layer in an FML will directly affect the bond strength of an FML. Past work has produced some interesting results. One body of work compared theoretical predictions to experimental results, with surprising results. Two finite-element analyses of the fractural loads of epoxy/aluminium lap shear joints and a theoretical model predicted that the maximum tensile loading before failure would increase with increasing thickness of the adhesive layer. Experimental results disagreed with these predictions. The fractural loading did not increase as adhesive thickness was increased and even decreased slightly [23, 24]. Results from the finite element analysis, theoretical models and various experiments are compared in Figure 2.9. The large variation in results from each study that contributed to Figure 2.9 suggest that definitive conclusions about the role of adhesive layer thickness cannot be drawn without further work. Figure 2.9 – Influence of adhesive layer thickness (h a ) on experimental and theoretical fracture loads of epoxy/aluminium single lap joints [22]. 11 Chapter 3: Method Tensile loading tests were conducted on the FML to determine which combination of adhesive film and surface treatment created the strongest bond between the aluminium and Curv. Three types of adhesive films were tested: Gluco, Xiro 23.601 and Xiro 23.110. Three types of surface treatment were also tested: untreated aluminium, anodised aluminium and etched aluminium. Further samples were prepared to conduct additional testing. In these secondary tests, two surface treatments and adhesive films were used; Gluco and Xiro 23.110, untreated and anodised aluminium. Justification of these choices can be found in the results and discussion section. Multiple layers of adhesive film were used in these tests; samples were made with 3 layers of adhesive film and 6 layers of adhesive film. 3.1 Construction 3.1.1 Materials The following materials were used in construction of the aluminium-Curv FMLs: • 2mm thick Aluminium 5005. • Xiro 23.110 and 23.601 adhesive films. • Gluco adhesive film. • 3mm thick CURV polypropylene laminate. • Isopropyl Alcohol. 3.1.2 Surface Treatment Three types of surface treatment were used on the aluminium. A third of the samples were left untreated, a third were etched using an acidic solution and a third were anodised. The samples with an untreated surface (cleaned with isopropyl alcohol) were made for two reasons. Firstly, a control group was required as a benchmark to measure the treated surfaces against, as the literature suggested treated samples would create a superior bond. Secondly, the manufacturer of both Xiro adhesive films claimed that their products bond with equal strength to both treated and untreated aluminium surfaces. Testing this claim was an important part of this thesis. A third of the samples were anodised at Fink & Co. [6]. To anodise the samples, the surface of the aluminium is given a matt finish using etching. The aluminium is then placed in an acidic electrolyte bath. An electric current is passed through the solution, creating an anodic film on the surface of the metal. 12 Another third of the samples were treated using a 5% sodium hydroxide solution to create an acidic etching on the surface. Sodium hydroxide (caustic soda) reacts with aluminium and water, releasing hydrogen gas. The aluminium takes an oxygen atom from the sodium hydroxide, which takes an oxygen atom from the water. Two hydrogen atoms are released in the process, leaving the surface with a deeper oxide layer and a rougher finish [5]. 3.1.3 Gluing Process Once the materials were acquired and the surfaces had been properly treated, isopropyl alcohol was used to give all surfaces a final clean, removing dust and foreign particles. A PHI PW220C-X4A (Figure 3.1) platen press was used to bond the CURV and the aluminium at 1000kPa and 155-160 °C. Curv, aluminium and adhesive film were layered on top of one another in a form suitable for double lap-shear tensile testing, as shown in Figure 3.2. The gluing process using the platen press was the same for the secondary set of tests, the difference being that multiple layers of adhesive film were placed on top of one another in the second set of tests (either 3 or 6). Figure 3.1 - PHI PW220C-X4A Gluing press with sample being prepared. 13 Figure 3.2 – Layering of materials in sample preparation. After a sample was cleaned and layered as shown in Figure 3.2, it was placed into the platen press and heated until the thermocouple gave a temperature reading of 155°C. A pressure of 1000kPa was applied to the sample via the platen press for 5 minutes. After 5 minutes at pressure, the press was rapid-cooled with water over 2-3 minutes until the temperature on the thermocouple read below 80°C, at which point the glue has set. Samples were then removed to cool further at room temperature. Previous work has shown that these conditions prevent delamination of the FML [28]. 3.1.4 Cutting Samples Double-lap shear samples were to have a width of 25mm. The samples made in the platen press were 275mm by 200mm rectangles. A rectangular or square shape was necessary to ensure uniform force and heat application. Therefore the samples had to be cut into 200mm by 25mm samples for lap shear testing. Given that both Gluco and Xiro adhesives have low bonding temperatures (155°C and 160°C respectively), a cutting method that did not generate large frictional temperatures was chosen. Furthermore, conventional cutting methods using a saw blade would tear apart the Curv laminate. Water jet cutting was the ideal solution to the problem as it uses a high pressure and a low temperature (below 100°C) jet of water to cut materials up to 45cm thick [8]. Additionally, Water jet cutting had been successfully employed by previous work that required cutting of aluminium-Curv FMLs. Serafin & Co., a water jet cutting company based in Queanbeyan, cut all samples used in the project. 3.2 Testing 3.2.1 Aramis and Instron Programming After all double-lap shear samples were made, a tensile test was proposed and developed. The parameters of the tensile testing were setup using methods defined in the Bluehill® software package, a software package available with the Instron 5500 mechanical loading machine. The Bluehill® software package is designed to be used in conjunction with the Instron 5500 and has predefined testing methods built in for mechanical testing. To perform lap shear tensile stress testing on the samples, a double-lap shear tension test method was selected from Adhesive Film Adhesive Film Curv Laminate Aluminium 5005 14 the software package. The tension test method is designed in accordance with relevant ASTM and ISO standards [26]. A spare sample was selected and tested to determine if the load cell and test parameters were sufficient. The sample was tested upon successfully. The pre-test sample failed between 4.5- 5kN, confirming that the largest load cell of 100kN would be necessary for safe testing (the next size down was 5kN). The feed rate of 5mm/min was sufficient to allow relevant data to be recorded by the Aramis. A specimen ready for testing in the Instron 5500 is shown in Figure 3.3. Figure 3.3 – Specimen prepared for testing in the Instron. Calibration of the Aramis involved setting the camera at the correct distance to allow full image capture of the bond zone. Measurement required one camera given the test involved 2D strain mapping. The image capture rate (frames per second) was determined based on the results from the pre-test. The pre-test sample experienced initial failure after 6-7mm of elongation. With the Instron tensile test method feed rate of 5mm/min, a sample was expected to take at least 60 seconds before initial failure. An image capture rate of 1 per second was deemed sufficient to capture enough data for strain analysis. The Aramis camera and analysis system takes measurements based on a tracking mechanism. To allow the Aramis to accurately measure strain/stress of a sample, a specific surface pattern must be present on the sample undergoing testing. The double lap shear samples tested for this thesis needed painting to create the pattern necessary for measurement. A painting 1S process was employed using a white primer as the base. Black aerosol paint was sprayed from a distance to create a stochastic pattern on the white base. Figure 3.4 shows a painted sample with a sufficient pattern for detection and measurement by the Aramis camera apparatus. Figure 3.4 – Stochastic paint and mask area of sample. The Aramis system locates each black dot (as seen in Figure 3.4) and tracks the movement of it during testing. The location of each dot is recorded in every image taken by the Aramis. Changes in location of each dot from image to image create the strain map. To separate the area of measurement from the background in the images, the user defines a mask region. The mask region covers the stochastic pattern and the area undergoing deformation. In the double lap shear experiments, the mask region was a rectangle covering the bond area, as seen in Figure 3.4. A more detailed description of the Aramis system and the benefits of using it for this thesis can be found in the appendix. 3.2.2 Microscopic Analysis Past work has extensively examined the microscopic surface topography of aluminium under various surface treatment methods [2,18,21]. Surface topographies have been ranked in order of success and the surface treatment methods that produce these topographies have been tested (see Figure 2.2). Microscopic analysis of tested samples was used to measure the success of the 3 types of surface treatments (anodising, etching and no treating) against the results from the literature. Similarities between surface topographies of the tested samples and the most favourable topographies from the literature were compared. Additionally, the success of each adhesive film in adhering to the surface of aluminium was studied. Observations were made regarding glue area coverage and direction of shear tear (in reference to [9]). Such analysis helped to Mask Area 16 understand why samples may have failed at the bond and why the glue may have not adhered to the surface of either the aluminium or Curv. 17 Chapter 4: Single Adhesive Layer Results The double-lap shear tests produced a range of different results. Most of the untreated samples did not survive the cutting process. Of the samples that were suitable for testing, 3 different types of failure were exhibited: adhesive, cohesive and structural. Analysis of the results was conducted using 3 methods. Firstly, the loading data collected by the Instron was analysed. Samples were also viewed under a microscope to examine the adhesive film failure and surface topographies. Finally, the footage captured by the Aramis was analysed to determine the shear and normal strain maps experienced by each sample up to initial failure. A discussion of the results is conducted at the end of this section along with justification for further testing. 4.1 Untreated Specimens Preparation of the double-lap shear samples was conducted under the assumption that all surface treatment and adhesive film combinations would achieve bonding. During construction a number of the samples failed, at 2 different stages. Firstly, one sample failed after removal from the gluing press. Additional samples failed during water jet cutting. Both of these results will be discussed in the following section, with further investigation using a microscope to analyse the adhesive failure of these samples covered in section 4.3. 4.1.1 Gluing Failure During the gluing process, one of the samples failed to bond. After the sample using Gluco adhesive film and untreated aluminium was removed from the gluing press it was observed that the bonding between the aluminium and Curv was unsuccessful. Closer inspection revealed that the majority of the surface of the aluminium had failed to bond with the adhesive film. With little effort and force, the weak bond between the aluminium and Curv was broken. Examination of the failed sample revealed that the Gluco adhesive film had bonded with the Curv polycarbonate, but exhibited poor adhesion to the untreated aluminium 5005, depicted in Figure 4.1. 18 Figure 4.1 – A failed untreated aluminium specimen. It is possible that during the construction and gluing process one of the many variables present may not have been correctly controlled, allowing the sample to fail. Another untreated aluminium and Gluco sample was constructed and glued. Visual inspection of the new sample did not indicate any deficiencies in the bonding of the aluminium and Curv. 4.1.2 Cutting Failure After one of the untreated aluminium samples failed during preparation, warning was given to Serafin & Co. that additional samples might fail during the water jet cutting process. It was requested that cutting be ceased immediately and contact made if a sample were to fail. During the cutting process, 3 additional samples failed. The samples with an untreated aluminium surface – Gluco, Xiro 23.601 and 23.110 – failed during the cutting process. As discussed in the method, the water jet cutting process does not reach a high enough temperature to melt either of the adhesive films (Gluco melts at 155°C, Xiro melts at 165°C). Failure therefore occurred either as a result of poor construction and gluing of all untreated samples or because bonding on an untreated aluminium surface is not possible [8]. Further investigation will be covered in the microscopic analysis in section 4.3. More than half of the etched aluminium Gluco samples failed during the cutting process. As the water jet cutting process does not heat Gluco above its melting point, failure must be caused by other factors. As with the untreated samples, an investigation will be discussed in section 4.3, looking at the aluminium surface and the adhesive film and comparing the failed samples with the etched Gluco samples which successfully bonded. 19 4.2 Instron Double-Lap Shear Tests Many of the samples exhibited prolonged failure zones (example shown in Figures 4.2) before complete failure. Common outcomes of the tensile testings were periods of extension where the loading application remained constant, as shown in Figure 4.2. These results can be largely attributed to processes other than de-bonding in the adhesive layer. A brief discussion and investigation will be developed in section 4.5 (for the most part, de-lamination of the Curv), but a detailed analysis is not relevant to understanding the bonding success and is therefore outside the scope of this thesis. Figure 4.2 – Extension vs. loading graph of alkaline Xiro 23.110 sample 1. Given that the goal of the experiment was to look at the failure point of the adhesive, not the ultimate failure of the double lap shear sample, analysis of the Instron results was taken up to the point of initial failure. Initial failure was defined as the first iteration (the INTSRON takes readings every 10 milliseconds) taken where the loading dropped by more than 1N, regardless of the cause of failure. The type of failure that occurred initially in each sample is vital to understanding the success of the bond. A brief description of the 3 possible failure methods is provided below: • Structural failure: Failure occurs within the Curv laminate. • Adhesive failure: Failure occurs due to de-bonding of the adhesive from the surface of either the Curv or aluminium. • Cohesive failure: Failure within the adhesive film. 4.2.1 Failure Type The type of initial failure in each sample was determined by observing the images captured by the Aramis. To find the time of initial failure, the Instron extension data was used. Given the feed rate of 5mm/min, the extension of each sample at initial failure could be converted to a 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 1 2 2 3 4 5 6 7 8 8 9 10 11 12 L o a d ( k N ) Extension (mm) Etched Aluminium Xiro 23.110 Sample 1 20 time and used to find the corresponding set of images taken by the Aramis. The images were analysed to determine which of the 3 types of failure were experienced by each sample. Types of failure were observed to be uniform amongst samples of the same adhesive/surface treatment. Figure 4.3 lists all the sample groups and the types of failure each underwent initially. Surface Treatment Adhesive Film Failure Type Gluco Adhesive Xiro 23.110 Cohesive Etched Xiro 23.601 Adhesive and Cohesive Gluco Structural Xiro 23.110 Cohesive Anodised Xiro 23.601 Cohesive Figure 4.3 – Failure methods for each sample set. A few observations can be made from the findings presented in Figure 4.3. Firstly, adhesive failure, as mentioned before, suggests the weakest point in the bond zone was the mechanical interlocking region between surface and adhesive. Two sample groups (Gluco and Xiro 23.601, both etched) failed adhesively. The corresponding Gluco and Xiro 23.601 anodised samples failed cohesively. From these initial observations, it can be concluded that the anodising process created a more successful surface topography for bonding than the etching process, in agreement with the literature. Of the three adhesive films, Gluco has produced the most promising results. Adhesive failure in the etched aluminium Gluco samples suggests surface treatment was the cause of initial failure. Similarly, in the anodised aluminium Gluco samples, structural failure suggests de- lamination of the Curv was responsible for initial failure. Xiro 23.110 failed cohesively with both surface treatments and Xiro 23.601 experienced both adhesive and cohesive failure. Cohesive failure in both Xiro adhesive films indicates that initial failure was caused by weakness of the adhesive films. At this stage of analysis, anodised aluminium with a Gluco adhesive film appears to have produced the strongest bond. This hypothesis will be compared to the quantitative data analysis from the Instron and Aramis measurements. 4.2.2 Stress at Initial Failure Having defined the initial failure point and the type of initial failure, the Instron data was collected and analysed to measure the stress at failure of each sample. Stress is given by the relationship between force and area, as shown in the formula below: Shear stress = F ÷ A Each double lap-shear sample had two bond regions with areas of 25mm x 75mm. Shear stress is calculated by taking the tensile load from the Instron data and dividing it by the area of the bonds in the sample. Figure 4.4 groups the samples into their respective surface treatment/adhesive film combinations to compare the stress of each sample set. 21 Figure 4.4 – Stress at initial failure of every tested sample. Xiro 23.601 is the weakest adhesive film, failing at lower stresses than all Xiro 23.110 and Gluco samples. Furthermore, the etched aluminium Xiro 23.601 samples produced unreliable bonds, with the 3 rd and 4 th samples failing at stresses approximately 4 times smaller than the 1 st and 2 nd samples. These results support the hypothesis that adhesive films have a maximum shelf life, after which the strength of the bond produced by the glue is compromised (The Xiro 23.601 was a few years past its recommended shelf life). Data for the other two adhesive films, Gluco and Xiro 23.110, was difficult to distinguish between. Statistical analysis (Figure 4.5) will aim to separate the two. The average stress of each set of samples was taken, along with a 95% confidence interval in an attempt to determine with statistical certainty which surface/adhesive combination produced the strongest bond. Formulas used in the statistical analysis can be found in the appendix. Figure 4.5 contains the results, with the mean of each sample set given along with the error bars indicating a 95% confidence interval. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1 2 3 4 S t r e s s ( M P a ) Sample Number Initial Failure Stress Etched Gluco Etched Xiro 23.110 Etched Xiro 23.601 Anodised Gluco Anodised Xiro 23.110 Anodised Xiro 23.601 22 Figure 4.5 – Average stress at initial failure of each sample type with confidence intervals. The statistical analysis proved that Xiro 23.601 is an unreliable choice of adhesive film. With 95% certainty, a bond produced with Xiro 23.601 will fail at stresses between 0MPa and 1.3Mpa. Xiro 23.601 fails at the weakest average stress of the three adhesive films with both etched and anodised surface treatments. Comparing the statistical analysis to raw data results in Figure 4.4, there is no conclusive evidence to suggest which adhesive film withstands the highest stress before failure. The confidence intervals of Gluco and Xiro 23.110 for both etched and anodised samples overlap. The mean stresses for these adhesive films are also very similar. The Instron data has proven that Xiro 23.601 is the weakest adhesive film, but has not separated the success of Gluco and Xiro 23.110 adhesive films. 4.3 Surface Topography An investigation of the surface of each tested specimen would provide further information in the success of the bond of each sample. The literature provided information on relevant features of the surface and adhesive film that would highlight the success of a bond. The 4 factors analysed in each sample under the microscope were: • Surface roughness and porosity. • Success of adhesion between glue and aluminium. • Success of adhesion between glue and Curv. -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 S h e a r S t r e s s M P a Adhesive and Surface Treatment Average Stress at Initial Failure S. Mean Upper CI Lower CI 23 • Failure method of bond. 4.3.1 Surface Conditions of Aluminium and Curv Firstly, the 3 types of treated aluminium surface and the surface of Curv were studied. Features compared were roughness depth and porosity. This was achieved by comparing the surface treatments to those found in the literature. Roughness depth and porosity directly relate to the success of mechanical interlocking between an adhesive and the surface of aluminium or Curv. Surfaces with varying depths and frequent holes adhere more successfully than smooth, flat surfaces [2]. Figures 4.6 – 4.8 show a clear difference between the surfaces of untreated aluminium 5005, anodised aluminium and etched aluminium. Untreated aluminium is smooth with fine abrasive damage in thin, straight parallel lines. Etched aluminium has small holes; anodised aluminium has more frequent and larger holes than the etched aluminium. The image taken on the right in Figure 4.8 is both in and out of focus, indicating that the anodised aluminium has deeper holes than the etched aluminium. This is in agreement with the literature, (refer to Figure 2.6) and indicates that anodised aluminium should promote the strongest bonds [2]. Figure 4.6 - Untreated aluminium, 5x resolution in left image, 50x in right image. Figure 4.7 - Etched aluminium, 5x resolution in left image, 50x in right image. 24 Figure 4.8 - Anodised aluminium, 5x resolution in left image, 50x in right image. Having established the superior surface treatment method to promote adhesion, the success of the 3 different adhesive films was investigated. As outlined in the method, the untreated aluminium samples all failed before testing could be conducted. A closer investigation of the surface revealed why bonding failed. All adhesive types – Gluco, Xiro 23.110 and 23.601 – exhibited little to no adhesion between surface and adhesive. No bonding occurred between untreated aluminium and both the Xiro adhesives. Figure 4.9 shows the untreated aluminium and Gluco sample. Small areas of Gluco have bonded to the aluminium, but the Gluco layer had failed to adhere strongly to the surface. Patches of Gluco were easily removed by hand. Figure 4.9 – Untreated aluminium Gluco sample. Figure 4.10 shows a Curv layer from a failed untreated aluminium sample. This piece of Curv is representative of all Curv from untreated aluminium samples. In these samples, adhesive films have completely bonded to the Curv and failed to bond to the aluminium. There are two 2S explanations as to why the untreated aluminium did not bond. Firstly, it is likely that an untreated aluminium surface is smooth and does not facilitate bonding. The second explanation is based on the melting temperature of Curv and the adhesive films. Curv is a polypropylene laminate and has a melting temperature of 160°C [19]. All samples were heated to a temperature of 155-160°C in the platen press. It is therefore possible that some melting occurred on the outer surface of the Curv that touches the adhesive film. This may have caused the adhesive and Curv to mix in their melted states, preventing the adhesive film from bonding to the untreated aluminium. This theory was part of the motivation for further testing, discussed in further detail in section 4.5. Figure 4.10 compares a Curv surface without glue (left image) to one has bonded with Gluco (right image). Note the difference in surface topography between the two pictures. The plain Curv has deep abrasions, whilst the Curv with Gluco is far smoother, suggesting a change in the surface of Curv that has been heated in the construction process. Figure 4.10 – Left image is 50x resolution of Curv surface. Right image is 50x resolution of Curv surface with Gluco. 4.3.2 Xiro 23.110 Having established that untreated aluminium samples unsuccessfully bonded, comparisons of etched and anodised samples were made for each adhesive film. The first adhesive film analysed under the microscope was Xiro 23.110. Figure 4.12 shows the etched Xiro 23.110 sample under the microscope, Figure 4.11 shows the cohesive failure of the anodised Xiro 23.110 sample. Both the anodised and etched Xiro 23.110 samples underwent clear cohesive failure. The tears in the adhesive run parallel to the direction of force application (similar to the sample in Figure 4.11) suggesting that significant plastic deformation occurred in the adhesive polymer during failure. One of the etched samples exhibited some adhesive failure. It can be concluded that failure in most Xiro 23.110 samples was caused by internal stress in the adhesive layer. 26 Figure 4.11 – Anodised Xiro 23.110 sample at 50x resolution. Figure 4.12 – Etched Xiro 23.110 sample, initial cohesive failure highlighted. 4.3.3 Xiro 23.601 Thorough analysis was a necessity to determine why the samples with Xiro 23.601 produced weak and unpredictable bonds. Etched aluminium samples 3 and 4 failed at loadings approximately 25% of samples 1 and 2 (approximately 1kN compared to 4kN+), a load far Cohesive Failure 27 smaller than all other samples (both etched and anodised), which failed at similar loadings (3.5-4.5kN). The first notable difference between etched aluminium samples 3 and 4 and the others was the success of the adhesion of Xiro 23.601 to the aluminium surface. As can be seen in Figures 4.13, adhesion has failed over the majority of the aluminium surface, with sparse, patchy sections of adhesive covering little of the surface. The results suggest that mechanical interlocking between adhesive film and the surface of the etched aluminium was poor. Etched samples 1 and 2 have experienced both adhesive and cohesive failure. Initially, the glue has failed cohesively, with ultimate failure occurring in an adhesive manner. Anodised Xiro 23.601 sample 4 exhibited successful adhesion between glue and aluminium surface. The adhesive sheared parallel to the direction of force application. Other anodised/Xiro 23.601 samples experienced differing failure methods in the bond. Samples 1, 2 and 3 underwent both cohesive and adhesive failure, with differing percentages of glue coverage on the aluminium surface. Figure 4.13 – Xiro 23.601 sample 3 resolution 50x, white patches are adhesive film. The microscopic analysis of Xiro 23.601 samples agreed with the Instron data. Results were highly inconsistent amongst common sample types, suggesting the quality of Xiro 23.601 has declined significantly after passing its recommended shelf life. 4.3.4 Gluco Etched Gluco samples were inconsistent. More than half of the samples failed during water jet cutting. Both of the samples that survived cutting and were used in lap shear tests failed adhesively. Microscopic analysis showed patchy glue coverage, similar to that shown in 28 Figure 4.13. Inconsistent results amongst common sample types suggest that etching aluminium is not reliable at creating a strong bond. All of the anodised aluminium Gluco samples failed structurally, preventing microscopic analysis of the adhesive film and surfaces. These samples successfully withstood stress at the interface between adhesive film and surface and within the adhesive film layer. Initial failure was a result of de-lamination of the Curv. 4.4 Aramis Strain Analysis Whilst the Instron measured the loading in each sample, and allowed the shear stress in the bond to be calculated, it did not accurately measure strain. The Aramis system recorded all movements and distortions in the sample, and used every deviation to calculate a 2D map of the strain for each image captured. The 2D strain maps highlight the failure points of a sample and can help to explain why some surface treatment and adhesive film combinations are more successful than others. The change in strain from the start of a test to initial failure highlights the weak points in a sample. Ideally, a perfect sample has strain distributed evenly across the width and length of the sample. This indicates that all materials within the sample are undergoing strain at the same rate, implying that the load is evenly distributed. A sample exhibiting these strain patterns will fail with a clean break perpendicular to the direction of force application. Given the number of variables in the production process, the different properties of each material within the composite and quality of these materials, perfect strain distribution is impossible to achieve. Initial results from the Instron indicate that a perfect bond was not achieved in any of the samples. It is therefore relevant to look for a strain pattern that is both realistic and indicative of a successful bond. There are a number of ways in which strain can measure the success of a bond. Strain is proportional to stress. Regions of high strain indicate sections within the sample that are experiencing high stress and are likely to fail first. A poorly bonded sample will have the highest strain over the bonds. Given that aluminium has a far greater tensile strength than Curv (>100MPa for aluminium [20], 12MPa for Curv [19]), in a well-bonded sample failure will occur in the Curv laminate far earlier than the aluminium. The Instron data concluded that both anodised and etched Xiro 23.110 and Gluco samples failed at similar loads. With a 95% confidence interval, there was not enough deviation to prove with statistical significance the superior sample subset. Analysis of the 2D shear and normal strain measurements taken by the Aramis aided in ranking the adhesive film and surface treatments. The first set of results looked at the samples that were statistically proven to have the weakest bond strength, samples with the Xiro 23.601 adhesive film. Samples 3 and 4 of the etched aluminium group failed at far smaller loadings than all other samples, as shown in Figure 4.4. 29 An analysis of the strain concentrations and failure points of these samples can be used as a benchmark to measure the success of other samples against. Clear shear strain concentrations can be seen along the length of the two bonds in Figure 4.14. Strain is low in both the Curv and aluminium. Etched aluminium samples with Xiro 23.601 have produced a poor bond; the initial application of loading has strained the full length of each bond leading to quick failure. Figure 4.14 – Aramis normal strain capture of an etched Xiro 23.601 sample. The other Xiro 23.601 exhibited more evenly distributed strain maps. Sample 1 of the etched aluminium set exhibited similar shear strain distributions to those of Figure 4.14. Sample 2 of the etched aluminium set showed more promising results with shear strain more evenly distributed through the Curv and bond regions. The anodised samples exhibited more even shear and normal strain distributions than the etched samples, supporting the analysis of the Instron tensile loading results. In these samples, strain was concentrated at the top of the bonds, similar to Figure 4.15. One of these samples experienced strain in only one bond, one de-bonded before initial failure and two of them experienced strain distributions like those in Figure 4.15. Similar to the conclusions drawn from sections 4.3 and 4.2, the normal and shear strains on the Xiro 23.601 samples were unpredictable. 30 All Xiro 23.110 and Gluco samples failed at similar tensile loads. Xiro 23.110 samples all failed cohesively, where as Gluco samples failed both adhesively and structurally, as discussed in section 4.3. It can therefore be stated that these two adhesive films did not create equally sound bonds. The Aramis footage shows that the etched Xiro 23.110 samples failed in a similar manner. Three of the samples produced high strain regions along the bond zones, with lower normal strain across the width of the sample than the Xiro 23.601 samples, as seen in Figure 4.15. Additionally, three of the samples experienced cohesive de-bonding before initial failure. The other sample had strain concentrated at the top of the bond zones extending into the Curv layer. Figure 4.15 – Aramis normal strain capture of an etched Xiro 23.110 sample. The four anodised Xiro 23.110 samples failed cohesively like the etched samples, but the measurements by the Aramis indicate different strain distributions. Shear and normal strain in all four samples was concentrated at the top of the bond zone, with little extension along the bond or into the Curv and aluminium layers. Maximum normal strain followed both bonds during cohesive failure, as shown in Figure 4.16. Furthermore, all samples failed cohesively as the tensile load was still increasing. De-lamination within the Curv layer was responsible for failure after the initial failure point. 31 Figure 4.16 – Aramis shear strain capture of an anodised Xiro 23.110 sample. The etched Gluco samples were inconsistent. Three of the samples failed during the water jet cutting stage. The two samples that survived cutting failed adhesively during tensile loading. The strain patterns measured by the Aramis suggest that the adhesive bond produced between an etched aluminium surface and Gluco adhesive film is similar to those in Figure 4.14. The first sample displayed clear strain concentrations along both bonds. The second sample had strain concentrated at the top of each bond zone until initial adhesive failure. The anodised Gluco samples exhibited the most even normal and shear strain distributions of the three adhesive films. All four of the samples failed structurally through de-lamination of the Curv. Three of the samples experienced initial failure at two points in the Curv close to the bonds; see Figure 4.17. All of the samples had strain concentrated in the Curv layer from application of loading until initial failure. In two of the samples, it was difficult to determine whether initial failure was cohesive or structural. In these samples cohesive and structural failure occurred within a second of one another. Set at a frame rate of one image per second, the Aramis footage could not accurately pinpoint the type of initial failure. 32 Figure 4.17 – Aramis normal strain capture of an anodised Gluco sample. The Aramis analysis suggests that anodised aluminium with Gluco adhesive film created the strongest bond. All other combinations failed either cohesively or adhesively, some with more favourable strain concentrations than others. 4.5 Scope for Further Testing From the first round of testing a few conclusions can be drawn. The microscopic analysis agreed with the literature that anodised aluminium creates the most favourable surface topography for mechanical interlocking to occur. Analysis of all Xiro 23.601 samples proved that the adhesive film was unreliable and produced bonds of inconsistent strength when used beyond the recommended shelf life. The statistical analysis of the Instron tensile loading of the Xiro 23.110 and Gluco samples could not separate the two groups. Strain maps captured by the Aramis assisted in distinguishing between the success of each surface treatment and adhesive film. Strain distributions of the Xiro 23.601 samples were inconsistent. From the analysis, it was clear that anodised aluminium Gluco samples produced the strongest bond. Maximum normal and shear strain was localised in the Curv layer in all of the samples. In other sample types, strain was concentrated on both the bond regions and the outer layer of the Curv. In these samples, complete failure occurred in the outer laminate layers of the Curv, close to the bonding zones, as seen in Figure 4.17. The outer layers of the Curv laminate may be compromised during gluing of samples. The gluing process applies heat and pressure to the sample to create the bond. The temperature of the press is between 155-160 °C. Taken from the manufacturer’s data, the minimum melting point of Curv is 160 °C [19]. During the gluing process, the adhesive film 33 melts to fill the pores on the surface of the aluminium. It is possible that the outer layers of the Curv may experience partial melting, given that the temperature approaches Curv’s melting point. These partially melted laminates layers may have weaker material properties than the rest of the Curv, explaining why failure is often observed here first. The adhesive film may fuse with the Curv, obstructing it from bonding to the surface of the aluminium. Given these results and hypotheses, a second round of testing was proposed. A few changes were made to take into account the successes and failures of the initial tests. Firstly, given the unreliability of Xiro 23.601 proven in the microscopic, Instron and Aramis analysis, it was removed from the second round of testing. The etched surface treatment was also removed, due to poorer results than the anodising treatment, with specific reference to Figure 4.3. Figure 4.3 lists the type of failure for each set of samples. Etched aluminium Gluco and Xiro 23.601 samples failed adhesively, the equivalent anodised aluminium samples did not. Further consultation with the manufacturer’s of Xiro 23.110 and other parties working with the adhesive film based in England strongly suggested that bonding is achievable without prior treatment of the surface. Increasing the thickness of the adhesive layer may allow the glue to both bond with the surface of the aluminium and fuse with the melted out layers of the Curv laminate. The next series of tests included anodised and untreated aluminium samples. Both Gluco and Xiro 23.110 adhesive films were tested again. In the second set of testing, 3 and 6 layers of adhesive film were tested. Referring to Figure 4.18, a total of 8 different sample types were to be tested. Five samples were made for each test combination. Surface Treatment Adhesive Film Number of Layers Number Of Samples 3 5 Gluco 6 5 3 5 Anodised Xiro 23.110 6 5 3 5 Gluco 6 5 3 5 Untreated Xiro 23.110 6 5 Figure 4.18 – Parameters for tests of the adhesive film thickness. 34 Chapter 5: Multiple Adhesive Layer Results Further testing was conducted using multiple layers of adhesive film. The goal of the secondary tests was to answer some questions brought up from initial tests and to distinguish between similar results. Namely; • To measure the viability of bonding on untreated aluminium surfaces, specifically with the Xiro 23.110 adhesive film (given the manufacturer’s claim that bonding on untreated aluminium was possible). • If bonding is possible with untreated surfaces, to measure the strength of the bonds and compare to bonds formed with an anodised aluminium surface. • To find which adhesive film - Gluco or Xiro 23.110 - produces a stronger bond, and the optimal thickness of the bond. 5.1 Untreated Samples All of the untreated aluminium samples successfully bonded during the gluing press. Initial observation suggested that the bond was of sufficient strength and that additional layers of adhesive film had allowed adhesion to occur on an untreated surface. Of the 20 samples produced with an untreated aluminium surface, 16 failed during water jet cutting. All of the samples with 6 layers of adhesive film failed. Four samples with 3 layers of Gluco survived, as did one sample with 3 layers of Xiro 23.110 adhesive film. Preliminary results suggested that increasing the thickness of the adhesive film did not aid in creating a bond on the surface of untreated aluminium. The samples that survived the water jet cutting stage were prepared for testing in the Instron. Loading a sample in the Instron required application of a small torque force about the major axis of the sample. This rotation is caused by the grips that can rotate a few degrees either side of their main axis due to the mechanism of their insertion and attachment. When tightening the grips and locking the sample in place, they tend to rotate slightly off axis when the final few turns of the vice are applied. This torque force was large enough to tear the bond in all of the untreated 3 layer Gluco samples. The untreated 3 layer Xiro 23.110 sample survived and was tested under tensile loading. Further investigation of the untreated samples will be covered in later sections, although it has been proven that bonding between the tested adhesive films and untreated aluminium 5005 is not possible. All but one of the anodised Gluco samples with 6 layers of adhesive film failed during cutting. The success of anodised Gluco samples in initial tests suggested bonding with this surface treatment and adhesive film was most favourable. Inspection of the failed samples revealed 3S the cause of the failure. Figure 5.1 shows one of the failed samples. One of the aluminium layers has a slight bend, where the other is flat. This difference in surface shape has prevented the Curv from bonding to both sheets of aluminium. Unfortunately, time constraints prevented another sample from being produced. Figure 5.1 – Anodised 6 adhesive layer Gluco sample. 5.2 Instron Double-Lap Shear Tests 5.2.1 Failure Type The testing method used in the single adhesive layer tests was reapplied in the multiple adhesive layer tests. Reusing the same test method ensured that consistency was maintained, allowing both sets of results to be accurately compared. In the multiple adhesive layer tests, as with the single adhesive layer tests, all double-lap shear samples were loaded until a 30% drop in loading occurred over a single iteration (0.1 sec). Analysis of the Instron data was taken up to the initial failure point, defined by a loading drop of more than 1N. The type of failure for each sample at this point was recorded using the Aramis footage and is listed in Figure 5.2. 36 Surface Treatment Adhesive Film Number of Layers Type of Failure 3 Structural Gluco 6 Structural 3 Structural Anodised Xiro 23.110 6 Structural 3 During Cutting Gluco 6 During Cutting 3 Cohesive + Adhesive Untreated Xiro 23.110 6 During Cutting Figure 5.2 – Initial failure types for each sample set. Initial observations suggested that favourable outcomes had occurred. All anodised samples experienced initial failure structurally through de-lamination of the Curv laminate. The Xiro 23.110 adhesive film failed cohesively on an anodised aluminium surface in the single layer tests. Thickening the adhesive layer of Xiro 23.110 has improved the strength of the bond, samples with thicker adhesive films failed structurally. The anodised Gluco samples also experienced structural failure, both in the single and multiple layer tests. Analysis of the Instron and Aramis data was undertaken to determine whether bond strength was improved. 5.2.2 Stress at Initial Failure Instron tensile loading data was converted to stress. Samples in the second round of testing were of the same dimensions as those in the first, 25mm x 200mm. The Instron was set with a 100kN loading cell and an extension rate of 5mm/min. Figure 5.3 groups the samples into their respective surface treatment/adhesive film combinations to compare the stress of each sample set. Figure 5.3 – Stress at initial failure grouped into sample types. 0.700 0.800 0.900 1.000 1.100 1.200 1.300 1.400 1 2 3 4 5 S t r e s s ( M P a ) Sample Number Stress at Initial Failure Anodised Xiro 3 Anodised Xiro 6 Anodised Gluco 3 Anodised Gluco 6 Untreated Xiro 3 37 Thickening of the adhesive layer produced stronger bonds, specifically the Xiro 23.110 samples. With a thicker adhesive layer, Xiro 23.110 samples produced stronger bonds than those with Gluco. It is difficult to determine the difference between 3 and 6 layers of adhesive film from the results in Figure 5.3. Again, statistical analysis of the data was undertaken to attempt to aid in differentiating between all sample types. Similarly to the single layer samples, mean, standard deviation, percentage error and confidence intervals were calculated. Figure 5.4 includes the results, with the mean and 95% confidence interval for each sample type shown (Formulas used in the statistical analysis are found in the appendix). Note that the anodised Gluco test with 6 layers and the untreated Xiro 23.110 test with 3 layers do not have confidence intervals as only one sample of each type made it to the testing stage. These two results harbour no statistical significance and are included for simple comparison. Figure 5.4 – Average stress at initial failure of each sample type with confidence intervals. Two conclusions can be drawn from the statistical analysis in Figure 5.4. Xiro 23.110, with a thicker layer of adhesive film than provided by the manufacturer, creates a stronger bond than Gluco on an anodised surface. With a single layer of adhesive film, Gluco creates the strongest bond, again on an anodised surface. Both the Gluco and Xiro 23.110 samples with 6 layers of adhesive film did not create stronger bonds than those with 3 layers. Therefore the optimum thickness of an adhesive layer is 1, 2 or 3 layers, for both Gluco and Xiro 23.110. By comparing the data in Figure 5.4 with Figure 4.5, we can determine the number of layers of adhesive film required to create the strongest bond. At this point, it has been established that an anodised aluminium surface is superior to 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 S t r e s s ( M P a ) Number of Layers and Surface Treatment Average Stress at Initial Failure (Multiple Adhesive Layers) S. Mean Upper CI Lower CI 38 an etched surface and an untreated surface. Figure 5.5 compares the statistical data for all anodised Gluco and Xiro 23.110 samples. Adhesive Number of Lower C.I. Mean Upper C.I. Failure 1 layer 1.028 1.106 1.183 Structural 3 layers 0.843 0.920 1.003 Structural Gluco 6 layers - 0.91 - Structural 1 layer 1.028 1.138 1.247 Cohesive 3 layers 1.100 1.140 1.176 Structural Xiro 23.110 6 layers 1.084 1.180 1.269 Structural Figure 5.5 – Comparison of confidence intervals of all Xiro 23.110 and Gluco tests. Increasing the adhesive layer thickness of anodised Gluco samples did not improve the strength of the bond. The range of 95% confidence for the 3 adhesive layer tests lies below the range of 95% confidence for the single adhesive layer tests. Therefore, the optimal thickness for an anodised Gluco samples is either one or two layers of adhesive film; the addition of further layers reduces the bond strength. Increasing the thickness of the adhesive layer in Xiro 23.110 samples did not conclusively increase the strength of the bond. Whilst the mean tensile stress increased from a single adhesive layer to 6 adhesive layers, the 95% confidence range of the single adhesive layer tests was wide enough to include the range of both multiple adhesive layer tests. Although the statistical analysis of the Instron data did not differentiate between the different adhesive thicknesses, the observational data did. Single layer Xiro 23.110 samples failed cohesively, but all the 3 and 6 layer samples failed structurally within the Curv layer. 3 and 6 adhesive layers of Xiro 23.110 created the strongest bond. 5.3 Aramis Strain Analysis An explanation of the Aramis testing procedure is outlined in the Method section. Further explanation of strain distributions and expected results was covered in section 4.4, which outlined what a perfect strain distribution looks like and realistic strain distributions to be expected in well-bonded samples. All of the anodised samples failed initially within the Curv layer. The Instron load measurements for these samples were similar to one another. The results of the Aramis strain footage were therefore vital to differentiating between the successes of the anodised aluminium multiple adhesive layer samples. Analysis of the strain distributions up to the initial failure point of each sample helped to rank them. All images used in this section showed either normal or shear strain maps measured by the Aramis. Shear strain highlights the regions of high stress within a sample. Normal strain can help to indicate the de- lamination of the Curv (see Figure 5.6). 39 Figure 5.6 – Normal strain of an anodised 6 adhesive layer Xiro 23.110 sample at initial failure. Samples with multiple adhesive layers showed promising results from the Aramis. All samples had maximum shear and normal strain distributions in the Curv layer at the top of the lap bond regions. In these samples, strain within the mask area was minimal and evenly distributed across the three material layers. Analysis of the strain map on the untreated aluminium Xiro 23.110 sample revealed poor adhesion. Furthermore, strain patterns of 3 and 6 adhesive layer samples were quite different and highlighted the differences in bonding strength between the two. As discussed earlier in this section, most of the anodised Gluco samples with 6 layers of adhesive film failed during the water jet cutting process. One sample did survive and was tested in the Instron. This sample failed through de-lamination of the outer layers of the Curv, supporting the theory that the Curv partially melts during gluing. Figure 5.7 shows the normal strain in the sample and highlights the point of initial failure. Whilst it appears to be failure in the bond, it is the outer layers of the Curv that are delaminating. 40 Figure 5.7 – Normal strain of an anodised Gluco 6 adhesive layer sample. The single untreated aluminium sample that survived until testing failed adhesively. Failure occurred in one of the bonds, with Aramis footage revealing normal strain concentrations extending down along the bond from the top of the adhesive fail point. Inspection of the sample after testing revealed an area of cohesive failure in the centre of the lap joint. All untreated samples in both sets of experiments failed with no adhesion between the surface and any type of adhesive film. It is therefore likely that the surface was accidentally roughened by abrasion during the manufacturing process, promoting some form of mechanical interlocking between Xiro 23.110 and the aluminium surface. As stated, 3 and 6 layers samples behaved differently under loading. The 3 adhesive layer samples, both Gluco and Xiro 23.110, exhibited maximum normal and shear strain in the Curv laminate. All samples failed initially in the Curv laminate at the top of or above the bonds, as highlighted in Figure 5.8. Initial Failure 41 Figure 5.8 – Normal strain of an anodised 3 adhesive layers Xiro 23.110 sample. Normal strain is concentrated on the Curv layer extending above the bond region. The mask zone had an even distribution of normal and shear strain (blue colouration if Figure 5.8), indicative of an ideal bond. Furthermore, the shear strain (Curv fails at 20% shear strain) at failure of the Curv laminate was in agreement with the literature [19]. The normal strain is dependent upon the severity of the de-lamination within the Curv. Higher normal strains are indicative of severer de-lamination of the Curv. Both shear and normal strains for each 3 adhesive layer sample are listed in Figure 5.9. Note that Gluco sample 2 and Xiro 23.110 sample 2 experienced initial de-lamination of the Curv outside the mask zone. The strain readings for these samples are somewhat inaccurate. 42 Adhesive Film Sample Number Shear strain (ε Y ) Normal Strain (ε X ) 1 19.2% 8.6% 2 - - 3 19.8% 7.1% 4 18.4% 6.3% Gluco 5 19.0% 12.5% 1 21.9% 6.2% 2 10.1% 5.5% 3 20.5% 8.5% 4 21.3% 14.8% Xiro 23.110 5 21.8% 6.7% Figure 5.9 – Shear and normal strain for adhesive film thickness tests. The 6 adhesive layer Xiro 23.110 samples experienced similar strain to the 3 layer samples. As discussed earlier, the 6 adhesive layer Gluco samples experienced problems with poor material quality, specifically the aluminium. Statistically, from the Instron analysis, the tensile strength of 6 and 3 adhesive layer Xiro 23.110 samples were very similar. The Aramis footage revealed differences between the number of adhesive layers. All samples failed within the outer layers of the Curv laminate. The bond zones had evenly distributed normal and shear strain, strain concentrations were at the top or above the mask zone. The accuracy of the shear and normal strain measurements taken by the Aramis was compromised by the structure of the composite. 6 layers of Xiro 23.110 adhesive film is 0.3mm thick, compared with 2mm thick aluminium sheets and 3mm thick Curv laminate. The thickness of the adhesive film may be too great, during construction the adhesive film melted and the pressure forced much of it outside above the bonds, highlighted in Figure 5.10. Figure 5.10 – Samples with 6 layers of adhesive film. 43 The build up of adhesive film above the bonds was enough to be captured by the Aramis camera. During tensile loading and extension the excess adhesive film separated from the top of the aluminium layers. Additionally, the glue, having weaker mechanical properties than both the Curv and Aluminium, experienced the largest normal and shear strains. Figure 5.11 shows the strain readings of a 6 adhesive layer sample, where maximum shear strain is experienced on the excess glue when it separates from the top of the aluminium. The large green area of shear strain indicates that the Curv laminate is experiencing greater tensile stress than the bonds and the aluminium. Figure 5.11 – Shear strain capture of a 6 adhesive layer sample. 5.4 Discussion Increasing the thickness of the adhesive film definitively increased the tensile strength of the bond between aluminium and Curv. However, thickening the adhesive layer does not promote adhesion between untreated aluminium and Curv. Untreated aluminium 5005 will not bond with Xiro 23.110 or Gluco. Only one of the 40 untreated aluminium samples survived until testing, suggesting that the tensile strength exhibited by this sample during testing was due to accidental roughening of the surface before the sample was glued. All other samples, similar to those in the first round of testing, failed during the water jet cutting process. 44 Two theories arose from the results of the multi-layer anodised sample tests. Firstly, all anodised samples failed initially within the Curv layer, suggesting that the bond created by both Xiro 23.110 and Gluco adhesive films produced a greater tensile modulus than recorded by the Instron. As the purpose of testing was to create a composite where tensile failure occurs in the Curv laminate before the adhesive bond, the actual tensile strength of the bond is not necessary. The second round of tests reinforced the second theory that the outer layers of the Curv laminate experience partial melting when in the gluing press. All samples failed in the outer layers; those that were left under loading until complete failure exhibited these failure patterns along the full length of the mask zone, as shown in Figure 5.12. Figure 5.12 – Multiple adhesive layer sample after testing in the Instron. 4S Chapter 6: Conclusions & Further Work 6.1 Conclusions This thesis investigated various methods of bonding Curv-aluminium FML. Three variables were measured: the surface treatment, the adhesive film, and the thickness of the adhesive film. Two sets of tests were conducted that determined the optimal set of conditions to create the strongest bond. The first round of tests proved that anodising aluminium 5005 surfaces created the most favourable surface topographies for adhesion. Etched surfaces were not as reliable as anodised surfaces, and untreated surfaces would not bond at all. Of the adhesive films tested, Gluco and Xiro 23.110 were statistically proven to produce stronger bonds than Xiro 23.601. Xiro 23.601 produced unreliable bonds, with wide variance in the results. Failure in the Curv laminate in the anodised Gluco samples suggested that this combination produced the strongest bond. The second round of tests, where the thickness of the adhesive film was increased, confirmed that with thicker adhesive layers, Xiro 23.110 created stronger bonds than Gluco. However, it was found that there is an optimal thickness for each adhesive film with thicker or thinner adhesive layers having weaker bond strengths. 3 layers of Xiro 23.110 created the strongest bond and were equal in strength to 1 or 2 layers of Gluco adhesive film. Anodised samples made with these adhesive films and specified thicknesses failed through delamination of the Curv. 6.2 Further Work FMLs have seen application in the commercial sector via the aerospace industry. Glass and aramid fibre based FMLs have been used in fuselage, wing and aircraft floor construction. In the automotive industry, aluminium-Curv FMLs aim to achieve similar material properties at a cheaper production cost for usage as panelling. Having determined the ideal surface treatment and adhesive film for bonding, further work would implement the results in current testing and production. Introduction of the work done in this thesis into future research involving aluminium-polypropylene FMLs could measure the changes to other properties of the composite. 46 Given that the work conducted in this thesis created a bond with greater tensile strength than the polypropylene laminate, testing of the change in impact properties would be an obvious extension of the work. Additionally, peel testing using the strongest bond conditions found in this paper would further the understanding of aluminium-polypropylene bonding. 47 Appendix A1 Statistical Analysis The goal of the statistical analysis of the loading data was to determine the significance of the results. A 95% confidence for the shear stress of a sample says that there is a 95% chance that a new sample tested will have a shear stress in the same range as the confidence interval. An example of how a confidence interval was calculated is set out below. The data used in the example is from all 5 Xiro 23.110 anodised aluminium 3 adhesive layer samples: Shear Stress = Sample 1: 1.096 MPa = Sample 2: 1.118 MPa = Sample 3: 1.205 MPa = Sample 4: 1.116 MPa = Sample 5: 1.154 MPa Mean = (1.096 + 1.118 + 1.205 + 1.116 + 1.154)/5 = 1.138 MPa Sample Standard Deviation = √[(1/4)*{(1.096 - 1.138) 2 + (1.118 - 1.138) 2 + (1.205 - 1.138) 2 + (1.116 - 1.138) 2 + (1.154 - 1.138) 2 }] = 0.043 MPa Standard Error = 0.043/(√5) = 0.019 MPa 95% Confidence Interval = [1.138 – (0.019*1.96)] to [1.138 + (0.019*1.96)] = 1.100 to 1.176 MPa The tables below contain all values calculated in finding the confidence intervals for the results. The first table contains values relating to Figure 4.5, the second table contains values relating to Figure 5.4. 48 Statistical data analysis used in Figure 4.5 in the table above. Sample Type Sample Mean (MPa) Sample Standard Deviation Standard Error Upper Confidence Interval Lower Confidence Interval Anodised Xiro 23.110, 3 layers 1.138 0.043 0.019 1.176 1.100 Anodised Xiro 23.110, 6 layers 1.177 0.105 0.047 1.269 1.084 Anodised Gluco, 3 layers 0.923 0.091 0.041 1.003 0.843 Statistical data analysis used in Figure 5.4 in the table above. A2 Aramis Strain Evolution The Aramis system has the unique ability to create 2D and 3D strain maps from images captured by a camera setup. By capturing multiple images during mechanical testing, the Aramis records an evolution of strain from steady state until failure. In terms of the work in this thesis, the Aramis allows analysis of the development of normal and shear strain in double-lap shear samples. Sample Type Sample Mean (MPa) Sample Standard Deviation Standard Error Upper Confidence Interval Lower Confidence Interval Anodised Gluco 1.106 0.056 0.040 1.183 1.028 Anodised Xiro 23.110 1.138 0.079 0.056 1.247 1.028 Anodised Xiro 23.601 0.835 0.118 0.083 0.998 0.672 Etched Gluco 1.021 0.112 0.079 1.176 0.865 Etched Xiro 23.110 1.059 0.063 0.044 1.146 0.973 Etched 23.601 Xiro Old 0.630 0.475 0.336 1.289 -0.028 49 Below are two series of images taken from an anodised aluminium Gluco single adhesive layer sample. These samples were observed to have ideal adhesive bonding. The first set of images captured from the Aramis looks at the development of normal strain, the second set looks at the development of shear strain. The first image is normal strain before testing. Images 2 and 3 were taken during tensile loading but before initial failure of the sample. Normal strain at this point is at a maximum at the top of the sample in what would be the Curv laminate. Image 4 was taken at initial failure, at this point the sample has begun to delaminate. Normal strain sharply increases – seen in image 5 - as the layers of Curv delaminate. S0 Image 1 depicts the shear strain before testing. Images 2 to 4 show the development of strain in the sample, in this case strain develops in the Curv laminate. Image 5 shows the shear strain at initial failure. Shear strain developed faster than normal strain. This was due to the direction of force application; shear strain was parallel to tensile loading, normal strain was perpendicular. Shear strain indicates which layers in the FML were experiencing the greatest tensile stress. Normal strain indicates where separation and delamination was likely to occur. S1 The examples of shear and normal strain above highlight the power of the Aramis. A simple image recording of a double-lap shear sample has provided accurate information regarding maximum shear stress during loading and indicated the point in the sample that will fail first. S2 Bibliography [1] Vlot, A. J., Gunnink, W., (2001), Fibre Metal Laminates: An Introduction, Kluwer Academic Publishers, Netherlands. [2] Botelho, E.C., Silva, R.A., Pardini, L.C. & Rezende, M.C. (2004), ‘Evaluation of adhesion of continuous fiber-epoxy composite/aluminum laminates’, Journal of Adhesion Science and Technology, vol. 18, no. 15-16, pp. 1799-1813. [3] Lawcock, G., Ye, L., Mai, Y. & Sun, C. (1997), ‘The effect of adhesive bonding between aluminium and composite prepreg on the mechanical properties of carbon-fiber- reinforced metal laminates’, Composites Science and Technology, vol. 57, no. 1, pp. 35- 45. [4] Ardakani, M.A., Khatibi, A.A. & Ghazavi, S.A. (2008), ‘A study on the manufacturing of glass-fiber-reinforced aluminum laminates and the effect of interfacial adhesive bonding on the impact behavior’, Society for Experimental Mechanics - 11th International Congress and Exhibition on Experimental and Applied Mechanics 2008, pp. 1948. [5] Caustic Soda. (2010), ‘Caustic Soda Application’. Available: http://www.caustic- soda.biz/caustic-soda-application. [Accessed 30 April 2011]. [6] Anodisers Association of Australia. (2010), ‘Anodising Information’. Available: http://www.anodising.org/information.php?How-is-it-done-2. [Accessed 20 June 2011]. [7] Chiang, M. Y. M. & He, J. (2002), Composite Part B, vol 33, pp. 461-470 [8] Lorincz, J. (2009), ‘Waterjets: Evolving from Macro to Micro’, Manufacturing Engineering, Society of Manufacturing Engineers. [9] Chen, M., Zhang, X., Huang, R. & Lu, X. (2008), ‘Mechanism of adhesion promotion between aluminium sheet and polypropylene with maleic anhydride-grafted polypropylene by γ-aminopropyltriethoxy silane’, Surface and Interface Analysis, vol. 40, no. 8, pp. 1209-1218. [10] David, E. & Lazar, A. (2003), ‘Adhesive bonding between aluminium and polytetrafluoroethylene’, Journal of Materials Processing Technology, vol. 143-144, no. 1, pp. 191-194. [11] Compston, P., Cantwell, W. J., Cardew-Hall, M. J., Kalyanasundaram, S. & Mosse, L. (2004), ‘Comparison of surface strain for stamp formed aluminum and an aluminum- polypropylene laminate’, Journal of Materials Science, vol. 39, no. 19, pp. 6087-6088. S3 [12] Romhány, G., Bárány, T., Czigány, T. & Karger-Kocsis, J. (2007), ‘Fracture and failure behavior of fabric-reinforced all-poly(propylene) composite (Curv®)’, Polymers for Advanced Technologies, vol. 18, no. 2, pp. 90-96. [13] Lopes, C. S., Remmers, J. J. C. & Gürdal, Z. (2006), ‘Influence of porosity on the interlaminar shear strength of fibre-metal laminates’, Collection of Technical Papers - AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, pp. 7848. [14] Davis, J. R. (1993), Aluminum and Aluminum Alloys, ASM International, Metals Park, OH. [15] Soltani, P., Keikhosravy, M., Oskouei, R. H. & Soutis, C. (2010), ‘Studying the Tensile Behaviour of GLARE Laminates: A Finite Element Modelling Approach’, Applied Composite Materials, pp. 1-12. [16] Karger-Kocsis, J., Varga J. (1996), ‘Effects of b-a transformation on the static and dynamic tensile behavior of isotactic polypropylene’, J. Appl. Polym. Sci., vol. 62, pp. 291–300. [17] Arrowsmith, D. J. (1970), Trans. Inst. Metal Finish, 48, 88. [18] Minford, J. D. (1977), Adhesives Age, vol 20(9), pp 41. [19] Propex Inc. (2003), ‘Curv – Technical Data Sheet C100A’. Available: http://www.curvonline.com/rightmenu/datasheets.html [Accessed 20 April 2011]. [20] Azom. (2008), ‘Aluminium Alloy 5005 – Properties, Applications, Fabrication, Machinability and Weldability of Alumi’, Available: http://www.azom.com/article.aspx?ArticleID=4244 [Accessed 20 April 2011]. [21] Kinloch, A. J. (1987), Adhesion and Adhesives, Chapman and Hall, Australia. [22] Adams, R. D. & Peppiatt, N. A. (1974), J. Strain Anal., 9, 185. [23] Bikerman, J. J. (1968), The Science of Adhesive Joints, Academic Press, New York, p. 273. [24] DeBruyne, N. A. (1944), Aircraft Eng., vol 16, pp. 115. [25] Ghosh, S. K. & Predeleanu, M. (1995), Materials processing defects, Elsevier Science BV, Netherlands, pp 265. [26] Instron. (2008), ‘Bluehill 2 Software’. Available: http://www.instron.us/wa/product/Bluehill2-Materials-Testing-Software.aspx [Accessed 10 August 2011]. [27] Reyes, G. & Cantwell, W. J. (2000), Comp Sci. Tech. 60, 1085. [28] Kim, J. K. & Thomson, P. F. (1990), J. Mater. Process. Technol., 22, 44. This thesis contains no material which has been accepted for the award of any other degree or diploma in any university. To the best of the author’s knowledge, it contains no material previously published or written by another person, except where due reference is made in the text. Stephen Handscombe 29 September 2011 © Stephen Handscombe Acknowledgements I would like to thank my supervisor Shankar Kalyanasundaram for his guidance and support throughout this project. His time and experience helped me get through some of the more difficult stages of the project and it has truly been a pleasure working with him. Your lighthearted approach boosted my spirits when in doubt! I would like to thank Anthony Sexton and Sudharshan Venkatesan for their continuous assistance and the time they sacrificed to help me along the way. Without your help I would not have been able to make my samples, work the Instron or setup the Aramis. Thank you to Dave Tychsen-Smith and Ben Nash for allowing me to use machinery in the engineering workshop and teaching me how to operate it. Thank you to Zbigniew Starchuski for the assistance you provided and the access you gave me to your materials lab. Finally I would like to thank my friends and family. Thank you for putting up with me over the last 8 months and for the encouragement you provided me along the way. Your support throughout my degree is greatly appreciated. but has not measured the strongest way to bond the aluminium and Curv.Abstract This thesis studied the bonding of aluminium-polypropylene (Curv) fibre metal laminates. Previous work has studied the mechanical properties and stamp formability. This paper investigated the bond strength created by different adhesive films and surface treatments by measuring stress and strain under loading. Double-lap shear samples were created and tensile loading was applied. . The testing and analysis produced a sample type with a bond that withstood larger tensile loads than the polypropylene laminate. with loading measurements taken and strain development recorded by image capture. ..............3 Surface Topography ................1 Curv ...................................................................................2......................................................................................................................................................................................1 Surface Conditions of Aluminium and Curv ..............................................................................................11 3......4 Cutting Samples...2 Microscopic Analysis .................................................................................1 1..................................................................................................................13 3.................................2...................1..................19 4.......................................................................................17 4..................................................1 1.............23 4.............................................3 Xiro 23...............................................................2 Surface Treatment.............................................................1....................................................................27 ..................................1 Failure Type..............10 Chapter 3: Method .......................................................................11 3..........................601 .....................................17 4...........3 Aluminium-polypropylene FMLs ....................................................................12 3..................11 3.18 4...............1 Construction ........................3......3................................4 2........19 4.................................................22 4.....13 3.......3..................................................1......................................................................................................................3 2..2 Testing......................................17 4.........................................................................................................................................................1 1...................................................................................1 Gluing Failure ................................................................11 3..2 Xiro 23..iv List of Figures ......4 Gluco....................5 2......................2 Surface Treatment and Adhesion.......................................Table of Contents Acknowledgements .........................3 2.................2 Cutting Failure .............................................................2..............................................................................................................2................................................................................................................1.................................3......................110 ...................6 2.............1 Aramis and Instron Programming ...............5 2..............................................................2 Mechanical Testing of FMLs .......................3 Gluing Process ......................................................3...........................................3 Aims and Contributions ..................2 Instron Double-Lap Shear Tests...........2 Chapter 2: Literature Review.................................................................1..............2 Stress at Initial Failure ...vi Chapter 1: Introduction ..............................................3 Thickness of Adhesive Layer ..............................................1 History of Fibre Metal Laminates ......................25 4................................................................1 Materials .................................................................................................1..............................13 3.1 Untreated Samples .....................................15 Chapter 4: Single Adhesive Layer Results ........2 Project Motivation....20 4........................................1 Background ...................3. iii Abstract ............................................................................................................3................................................................26 4............................................................................................................................................................................ .............................................................................................................................1 Failure Type.......................................................................4 Discussion ..........................................................34 5................................................................................................................................................43 Chapter 6: Conclusions and Further Work .........................3 Aramis Strain Analysis .......................................47 A2 Aramis Strain Evolution.........................................2.2 Instron Double-Lap Shear Tests...................................................48 Bibliography .......................................................2 Stress at Initial Failure ...................................................45 6...35 5.............................................................................................................32 Chapter 5: Multiple Adhesive Layer Results ................................2............45 Appendix..35 5..1 Conclusions .................................36 5..........................47 A1 Statistical Analysis ...........38 5..5 Scope for Further Testing ...........................................................45 6.......................................................................................................................................................................................................4 Aramis Strain Analysis .......................................................................................34 5...4...............................................................................................................................................................................................................................................................................................................2 Further Work...........1 Untreated Samples .....................................................52 ..................................28 4... 27 Figure 4....2 – Initial failure types for each sample set............................ 50x in right image............1 – A failed untreated aluminium specimen.110 sample ......2 – Extension vs....2 – Ranking of Surface topographies in terms of peel energy ...Untreated aluminium 2024-T3 ..15 – Aramis strain capture of an etched Xiro 23. grouped into sample types..........36 Figure 5..........35 Figure 5..................601 sample 3 resolution 50x...............Etched aluminium.....Anodised aluminium...................8 Figure 2.6 Figure 2........ Phosphoric acid anodised surface topology (right) .............................1 – Anodised 6 adhesive layer Gluco sample.......... Right image is 50x resolution of Curv surface with Gluco ................8 Figure 2...............................5 – Average stress at initial failure of each sample type with confidence intervals...................................................13 – Xiro 23....................SEM photographs of fracture surface with 0% γ-APS concentration ..............33 Figure 5.7 ..12 – Etched Xiro 23........Chromic acid anodised aluminium ....... 5x resolution in left image......17 – Aramis strain capture of an anodised Gluco sample ....7 Figure 2.............................................................18 – Parameters for tests of the adhesive film thickness.............12 Figure 3......................26 Figure 4......................... 5x resolution in left image...........29 Figure 4.................................3 .9 – Influence of adhesive layer thickness (ha) on experimental and theoretical fracture loads of epoxy/aluminium single lap joints .....................5 ..........110 sample at 50x resolution ..........1 ...110 sample...................36 .24 Figure 4..........................................................................SEM photographs of fracture surface with 3% γ-APS concentration ....................2 – Layering of materials in sample preparation...........1 .............................22 Figure 4...Untreated aluminium........6 – Chromic acid anodised surface topology (left)....13 Figure 3...............................................26 Figure 4..............................................................601 sample ...........................................18 Figure 4..............................31 Figure 4....9 Figure 2.......19 Figure 4............4 ......... white patches are adhesive film....................3 – Stress at initial failure.................6 ...............11 – Anodised Xiro 23..........Load-Displacement curves of Curv specimens .....14 Figure 3.......................20 Figure 4....110 sample ..... loading graph of alkaline Xiro 23................10 – Left image is 50x resolution of Curv surface......................................................14 – Aramis strain capture of an etched Xiro 23........3 – Failure methods for each sample set .........................................23 Figure 4......8 ..............................5 Figure 2.........24 Figure 4.....................4 – Stress at initial failure of every tested sample.....................3 – Specimen prepared for testing in the Instron.4 – Stochastic paint and mask area of sample ...........................................................................9 – Untreated aluminium Gluco sample..8 ......9 Figure 2.25 Figure 4.......Sulphuric chromic acid etched aluminium ......................................... 5x resolution in left image....21 Figure 4............. 50x in right image .................................. initial cohesive failure highlighted .........16 – Aramis shear strain capture of an anodised Xiro 23.............................................7 Figure 2...................15 Figure 4................................23 Figure 4..............7 ................................110 sample 1....PHI PW220C-X4A Gluing press with sample being prepared........10 Figure 3..List of Figures Figure 2..............30 Figure 4...........................32 Figure 4............................ 50x in right image................ ..........44 ................40 Figure 5...............................8 – Normal strain of an anodised 3 adhesive layers Xiro 23.................37 Figure 5...4 – Average stress at initial failure of each sample type with confidence intervals..............42 Figure 5............10 – Samples with 6 layers of adhesive film...................................110 sample ...........5 – Comparison of confidence intervals of all Xiro 23..........................................12 – Multiple adhesive layer sample after testing in the Instron............41 Figure 5............9 – Shear and normal strain for adhesive film thickness tests.................110 and Gluco tests .............................42 Figure 5.......43 Figure 5............................................6 – Normal strain on an anodised 6 adhesive layer Xiro 23...................110 sample at initial failure ..............Figure 5....38 Figure 5........7 – Normal strain of an anodised Gluco 6 adhesive layer sample ........................11 – Shear strain capture of a 6 adhesive layer sample...39 Figure 5.... 1 Background A Fibre Metal Laminate (FML) is a composite material made by layering sheets of high strength aluminium alloy with a fibre/epoxy composite. Structures made out of FMLs have been designed considering ideal mechanical and bonding properties determined by the Classical Laminate Theory. The development of FMLs began in the late 1970’s as one of a number of new materials to be utilised in the growing aerospace and high technology industries [1]. [1] FMLs have many advantages over existing alloys and metals. Exhibiting high stiffness-to-weight ratio. . FMLs can be categorised into 3 groups: Aramid-FibreReinforced Aluminium (ARALL). Glass-Fibre-Reinforced Aluminium (GLARE) and Carbon-Fibre-Reinforced Aluminium (CARAL) Laminates [2]. 1. adhesive thickness and surface treatments will provide a greater understanding of the limitations of FMLs. the fibre laminate and the bonding agent.2 Project Motivation To develop a comprehensive understanding of the properties and limitations of an FML. achieving perfect bonding conditions is difficult.Chapter 1: Introduction 1. In practice. The adhesion between the composite laminate and aluminium is vital to the overall performance of the FML. A greater understanding of bonding limitations will be achieved through testing and analysis. strong resistance to crack propagation and fatigue and high impact resistance. This theory assumes perfect bonding conditions between aluminium and fibre laminate layers. There have been numerous studies conducted on the mechanical properties of FMLs. there must be an understanding of the mechanical properties of the metal. and how different adhesive agents change the performance and mechanical properties of the FML [2][3][4]. During the production process. if not impossible. numerous complications can lower the quality of the bond. Understanding the strengths and the weaknesses of different bonding agents. It is therefore imperative that the mechanical properties of the bonding agent are known. Past research has focused both the aluminium and the fibre laminates. 110 adhesive films. Produce an aluminium-polypropylene FML where failure under loading occurs in the Curv laminate before the bond. Tensile tests followed by stress and strain analysis will measure bonding success. • • .1. Previous work has investigated how material properties of FMLs are improved or diminished by different bonding processes.3 Aims and Contributions The aim of this thesis is to determine the tensile strength and peeling strength of adhesive films used with different surface treatments in aluminium-polypropylene FMLs. This research will address the following unique contributions to the field of FMLs: • • Measure the tensile strength of Gluco and Xiro 23.601 and 23. Test the effect the thickness of the adhesive film has on the strength of a bond. but has neglected to measure the mechanical properties of the adhesives used in the process. Test 3 different adhesive films and 3 different surface treatments to find the combination that yields the highest shear strength of an aluminium-polypropylene FML. In these tests. lithium aluminium alloys and 6013 . The superior material properties of FMLs were proven in a series of tests conducted by Airbus from 1990 – 1994 [1]. They had easy impact identification. Finally. For example. wing and internal floor construction.1 History of Fibre Metal Laminates Fibre Metal Laminates are a part of the rapidly growing sector of the composite material science industry. They had excellent resistance to crack growth. a Dutch chemical company [1]. Their adoption by the commercial sector over traditionally used metals and alloys is not widespread. FMLs were made with either a glass-fibre reinforced laminate or an aramid fibre reinforced laminate with an aluminium alloy. This chapter investigates the initial production of FMLs and the development of them over the last few decades. carbon fibre reinforced (CARAL) and aramid fibre reinforced (ARALL). where FMLs were being used for aircraft fuselage. propagation and material fatigue. glass-fibre reinforced aluminium was compared with common aluminium alloys then used in aircraft.Chapter 2: Literature Review The aim of this chapter is to introduce the reader to the composite material industry. The materials used in the FML from this thesis will be introduced. Initially. FMLs were first commercialised and patented in the 1980’s by AKZO. They had excellent burn-through resistance. Permanent damage could be found with visual inspection due to plastic deformation. Their high-speed impact properties were far superior to monolithic aluminium. Development of FMLs began in the late 1970’s. ranges of tests were conducted on FMLs to compare their material properties to those of monolithic aluminium. It will then focus on the key materials used in their production. Commercialisation began in the late 1980’s. The aluminium layers were treated with an acid based anodising process to promote adhesion of the laminate. During initial research and the first years of commercialisation. FMLs can be categorised into 3 groups separated by the material used in the laminate layer: glass fibre reinforced (GLARE). specifically fibre metal laminates. with room for growth. 2. Airbus currently uses FMLs for fuselage and internal plane part construction [7]. looking closely at the superior material properties and limitations of each. a look at previous work on aluminium surface treatments and adhesive layer thickness will conclude the literature review. A number of favourable properties of FMLs were found: • • • • • They were up to 20% lighter than aluminium. It was not until 2000 that work began on aluminium-polypropylene FMLs [27].alloys. Such testing aimed to prove that FMLs exhibit superior mechanical properties to monolithic metals. Good adhesion bonding showed better resistance to low velocity impacts with corresponding contact forces 25% higher than specimens with poor adhesive bonding [4]. where weight savings and impact damage reductions are important issues. was significantly influenced by the success of the interfacial bonding. Previous work has looked at the viability of stamp forming FMLs. Barrels made of the FML and the aluminium alloys were constructed. With superior impact properties and a lower density than that of monolithic aluminium. whilst the barrels made from other materials had to be repaired after significant crack growth. The aim of the tests was to replicate the damage to the fuselage of an operational aircraft. their alloys and standalone fibre based laminates. impact behaviour. strain increased with increasing tool radii [11]. increasing the tool radii did not increase the strain in the aluminiumpolypropylene laminate. one of the favourable properties of FMLs. Poor interfacial adhesion resulted in a 10% reduction in the interlaminar shear strength in one body of work [3]. . Furthermore. FMLs would serve as a favourable replacement to the current metals and alloys used in automobile body panels. Earlier work has demonstrated that adhesive choice and bonding conditions have a significant effect on the tensile strength of FMLs. Additionally. Recent study has indicated that aluminium-polypropylene laminates have exhibited excellent specific mechanical properties and superior impact performance to monolithic aluminium. Additionally. Materials with the described properties have potential application in the automotive industry. Hammers. Automobile body panels are most commonly stamp formed [25]. Testing on aluminium-polypropylene FMLs was conducted comparing the surface strain of stamp formed samples to that of stamp formed monolithic aluminium. the study of different combinations for specific applications is vital. and will be discussed in greater detail in the following sections. materials and material grades. Current work involves the testing of the stamp formability of aluminium-polypropylene FMLs. In the monolithic aluminium samples. The aluminium-polypropylene laminate exhibited lower strain. extensive mechanical testing has been conducted under varying conditions and parameters. cracks and corrosion damage respectively. Impact tests conducted under ASTM D7136 guidelines found that impact damage size is greater with poor interfacial adhesion. at 1000 per week over approximately 4 years. and 100 000 fatigue tests were conducted on all barrels. saws and milling machines were used to replicate dents. 2.2 Mechanical Testing of FMLs To fully understand the limitations and potential applications of FMLs. adhesive agents. The FML barrel survived the testing. given the vast number of surface treatments. impact resistant composite designed by Propex Incorporated. Curv has potential applications in the automotive industry due to its recyclability. approximately 30/kJ/m2 [16]. It is an excellent candidate for usage with aluminium in FML production. a self-reinforced 100% polypropylene woven-fibre laminate. Success in all three areas is vital to creating a strong bond.Load-Displacement curves of Curv specimens [12]. To create a successful bond. the adhesive layer must have some way in which it can adhere to the surface of both the aluminium and the laminate. Figure 2. is a lightweight. 2.1 highlights the poor consolidation of Curv during tensile loading. and impact resistance (impact strength increases as temperatures decrease). the adhesive film/agent used and the choice of curing process. Mechanical interlocking is the process  .2/kJ/m2) [12] . resulting in multiple failure stages.1 . low density (≈0.3 Aluminium-polypropylene FMLs 2.47. due to somewhat poor consolidation during mechanical loading.3. surface treatment. where reducing weight and increasing impact resistance are primary goals.3. a subsidiary of British Petroleum. It was found that Curv does not behave as an elastic material.2. Figure 2. Past work has shown that there exist three key factors that influence the success of the interlaminar bonding of FMLs. The fracture energy of Curv .9g/cm3). The failure and fracture behaviour of Curv has been tested to determine the limitations of the material and its viability in the FML and automotive industries.2 Surface Treatment and Adhesion The focus of this thesis will be research regarding the bonding of FMLs.1 Curv Curv.is more than 50% larger than that of unreinforced polypropylene. but not to a significant extent [18]. The introduction of an abrasive agent to roughen the surface (mechanical roughening) followed by cleaning with an ethanol-based liquid has been proven to increase the strength of interlaminar bonding between aluminium and polytetrafluroethylene [10]. Additionally. high-angle pyramids with dendrites gives surfaces similar peel strength.2 for extensive rankings [17].2 – Ranking of Surface topographies in terms of peel energy [21]. mechanical roughening methods (such as gritblasting) do not produce desirable surface topographies for establishing mechanical interlocking with an adhesive.where an adhesive agent solidifies within the oxide layer on the surface of a substrate. The substrate needs to be sufficiently rough and irregular to allow the adhesive to interlock. . The interlocking of an adhesive into the irregularities of the surface of the material is the key source of adhesion in a bond. Past work has shown that mechanical roughening improves the joint strength. High-angle pyramid structure with dendrites and club-headed nodular structure produce peel energy strengths more than 3 times those of a flat smooth surface when bonding copper foil to an epoxy laminate. Chemical pre-treatments are an excellent method of roughening surfaces on the micro-level. see Figure 2. Literature on measuring the peel energy of various surface topographies has shown that there are two types of surface topographies that promote the strongest mechanical interlocking. There are two methods to create a surface suitable for an adhesive to mechanically interlock: mechanical roughening and chemical pre-treating. The other surface topography. Roughness produced on the micro-level has been shown to produce successful mechanical interlocking [17]. Figure 2. This can be attributed to the poor success of mechanical interlocking when roughness is produced on the macro-level (as with grit-blasting and other mechanical roughening techniques) [18]. . Chromic acid anodised aluminium [2]. These pores and whiskers are somewhat similar to one of the favourable surface topographies listed in Figure 2.5).2. Figure 2. Figure 2. Etching and anodising pre-treatments increase the thickness of the oxide layer on the surface of aluminium. the club-headed nodular structure. Chromic acid anodising is another widely used method.2. creating a network of pores and whiskers (1-2µm thick.4 .Untreated aluminium 2024-T3 [2]. . see Figures 2.3 . Sulphuric chromic acid etching is the most commonly used method of treating the surface of aluminium.3 .Chemical pre-treatments of the surface are more successful at improving bond strength. A bond produced under theses conditions will have strength between that of the adhesive and that of the oxide layer. Furthermore. Phosphoric acid anodised surface topology (right) [21]. both processes lead to higher surface energies than those of untreated aluminium 2024-T3 [2]. Etching and anodising processes create a system in which mechanical interlocking can occur between the aluminium and an epoxy resin (the adhesive) [2]. porous surface topography.6 – Chromic acid anodised surface topology (left).  . but with shallower pores. This is because the anodising process is easier to control and therefore creates a more consistent surface topography than etching.Sulphuric chromic acid etched aluminium [2]. Other work has shown that the anodising process is less susceptible to mechanical damage than the etching process [14]. The etching process creates a similar surface topography.5 . Anodising aluminium creates a deep. Figure 2.Figure 2.6 highlights the differences in the surface topographies of anodised and etched aluminium surface treatments. Adhesive film applied to an anodised surface typically penetrates to the bottom of these pores [21]. Anodising and etching processes produce a surface finish approximately 20% rougher than that of untreated aluminium. Figure 2. Heating temperature. The fibres in Figure 2. when applied to the surface of untreated aluminium. indicative of larger shear loadings before failure than those experienced by the sample in Figure 2. The introduction of a silane compound is a method of promoting adhesion between untreated aluminium and polypropylene. greatly improves the lap-shear strength between aluminium and polypropylene. Previous work has investigated the success of primers at increasing the bond strength of untreated monolithic aluminium [9]. Maximum lap-shear strength was achieved at a γAPS concentration of 3% [9].Along with surface treatments.8 . curing pressure and time are factors that influence the strength of the completed bond [10].8 show the fracture surface on the aluminium.SEM photographs of fracture surface with 3% γ-APS concentration [9]. Porosity in the glue significantly reduces the interlaminar shear strength of FMLs. Parameters of the curing process directly affect the final strength of the bond between layers.SEM photographs of fracture surface with 0% γ-APS concentration [9].7. Figure 2. cooling rate. The fibres in Figure 2. The use of primers to increase adhesion and environmental resistance of aluminium surfaces is a widely established practice. Figures 1.7.7 .8 fail parallel to the direction of force application. primers can increase the adhesive strength of a metallic surface. γ-aminopropyltriethoxy silane (γ-APS).7 and 1.8 exhibited far greater plastic deformation than those in Figure 2. Figure 2. Primers are typically used to promote adhesion on smooth surfaces that would not otherwise bond to adhesives. Mechanical testing showed a drop in interlaminar shear  . 9 suggest that definitive conclusions about the role of adhesive layer thickness cannot be drawn without further work.9. Two finite-element analyses of the fractural loads of epoxy/aluminium lap shear joints and a theoretical model predicted that the maximum tensile loading before failure would increase with increasing thickness of the adhesive layer.strength of up to 30% when using samples with a porous adhesive layer due to failure of the curing process [13]. theoretical models and various experiments are compared in Figure 2. The large variation in results from each study that contributed to Figure 2. 2. Experimental results disagreed with these predictions.9 – Influence of adhesive layer thickness (ha) on experimental and theoretical fracture loads of epoxy/aluminium single lap joints [22]. One body of work compared theoretical predictions to experimental results.3. . It is reasonable to assume that the thickness of an adhesive layer in an FML will directly affect the bond strength of an FML. little has been done measuring the role of the thickness of the adhesive agent. Results from the finite element analysis. Past work has produced some interesting results.3 Thickness of Adhesive Layer Compared to the body of work looking at the role of surface topography on successful adhesion. The fractural loading did not increase as adhesive thickness was increased and even decreased slightly [23. Figure 2. with surprising results. 24]. An electric current is passed through the solution. samples were made with 3 layers of adhesive film and 6 layers of adhesive film. A third of the samples were left untreated. Justification of these choices can be found in the results and discussion section. the surface of the aluminium is given a matt finish using etching.110. Firstly. Xiro 23. Three types of surface treatment were also tested: untreated aluminium. Gluco and Xiro 23. Isopropyl Alcohol.1 Construction 3.110 and 23. To anodise the samples.Chapter 3: Method Tensile loading tests were conducted on the FML to determine which combination of adhesive film and surface treatment created the strongest bond between the aluminium and Curv. Testing this claim was an important part of this thesis. Gluco adhesive film. The aluminium is then placed in an acidic electrolyte bath. [6]. a control group was required as a benchmark to measure the treated surfaces against. two surface treatments and adhesive films were used. a third were etched using an acidic solution and a third were anodised.601 and Xiro 23. The samples with an untreated surface (cleaned with isopropyl alcohol) were made for two reasons. as the literature suggested treated samples would create a superior bond. Secondly. Multiple layers of adhesive film were used in these tests. creating an anodic film on the surface of the metal. . Xiro 23. Three types of adhesive films were tested: Gluco. 3. In these secondary tests. the manufacturer of both Xiro adhesive films claimed that their products bond with equal strength to both treated and untreated aluminium surfaces.110.2 Surface Treatment Three types of surface treatment were used on the aluminium. Further samples were prepared to conduct additional testing.1. 3mm thick CURV polypropylene laminate.1 Materials The following materials were used in construction of the aluminium-Curv FMLs: • • • • • 2mm thick Aluminium 5005.1.601 adhesive films. A third of the samples were anodised at Fink & Co. 3. anodised aluminium and etched aluminium. untreated and anodised aluminium. Two hydrogen atoms are released in the process. Curv. 3. Sodium hydroxide (caustic soda) reacts with aluminium and water. which takes an oxygen atom from the water.Another third of the samples were treated using a 5% sodium hydroxide solution to create an acidic etching on the surface.1) platen press was used to bond the CURV and the aluminium at 1000kPa and 155-160 °C. isopropyl alcohol was used to give all surfaces a final clean.1 .3 Gluing Process Once the materials were acquired and the surfaces had been properly treated. The gluing process using the platen press was the same for the secondary set of tests. aluminium and adhesive film were layered on top of one another in a form suitable for double lap-shear tensile testing. . Figure 3. The aluminium takes an oxygen atom from the sodium hydroxide. leaving the surface with a deeper oxide layer and a rougher finish [5]. as shown in Figure 3. the difference being that multiple layers of adhesive film were placed on top of one another in the second set of tests (either 3 or 6).2. removing dust and foreign particles. releasing hydrogen gas. A PHI PW220C-X4A (Figure 3.PHI PW220C-X4A Gluing press with sample being prepared.1. Previous work has shown that these conditions prevent delamination of the FML [28].1 Aramis and Instron Programming After all double-lap shear samples were made. a cutting method that did not generate large frictional temperatures was chosen. Therefore the samples had to be cut into 200mm by 25mm samples for lap shear testing. Water jet cutting had been successfully employed by previous work that required cutting of aluminium-Curv FMLs.2 Testing 3. Additionally. 3.1. Serafin & Co. a double-lap shear tension test method was selected from . 3.4 Cutting Samples Double-lap shear samples were to have a width of 25mm. at which point the glue has set. it was placed into the platen press and heated until the thermocouple gave a temperature reading of 155°C. a tensile test was proposed and developed. After a sample was cleaned and layered as shown in Figure 3.Adhesive Film Aluminium 5005 Adhesive Film Curv Laminate Figure 3. conventional cutting methods using a saw blade would tear apart the Curv laminate. Given that both Gluco and Xiro adhesives have low bonding temperatures (155°C and 160°C respectively). The Bluehill® software package is designed to be used in conjunction with the Instron 5500 and has predefined testing methods built in for mechanical testing.2. Samples were then removed to cool further at room temperature. the press was rapid-cooled with water over 2-3 minutes until the temperature on the thermocouple read below 80°C. a water jet cutting company based in Queanbeyan. cut all samples used in the project.2 – Layering of materials in sample preparation. a software package available with the Instron 5500 mechanical loading machine. Furthermore.2. To perform lap shear tensile stress testing on the samples. A pressure of 1000kPa was applied to the sample via the platen press for 5 minutes. After 5 minutes at pressure. The parameters of the tensile testing were setup using methods defined in the Bluehill® software package.. The samples made in the platen press were 275mm by 200mm rectangles. Water jet cutting was the ideal solution to the problem as it uses a high pressure and a low temperature (below 100°C) jet of water to cut materials up to 45cm thick [8]. A rectangular or square shape was necessary to ensure uniform force and heat application. Figure 3. confirming that the largest load cell of 100kN would be necessary for safe testing (the next size down was 5kN). With the Instron tensile test method feed rate of 5mm/min. a specific surface pattern must be present on the sample undergoing testing. A painting . Measurement required one camera given the test involved 2D strain mapping. The pre-test sample failed between 4.the software package. Calibration of the Aramis involved setting the camera at the correct distance to allow full image capture of the bond zone. A specimen ready for testing in the Instron 5500 is shown in Figure 3. To allow the Aramis to accurately measure strain/stress of a sample. The sample was tested upon successfully. The Aramis camera and analysis system takes measurements based on a tracking mechanism. The image capture rate (frames per second) was determined based on the results from the pre-test. a sample was expected to take at least 60 seconds before initial failure. The pre-test sample experienced initial failure after 6-7mm of elongation.55kN.3 – Specimen prepared for testing in the Instron. A spare sample was selected and tested to determine if the load cell and test parameters were sufficient. The double lap shear samples tested for this thesis needed painting to create the pattern necessary for measurement. The tension test method is designed in accordance with relevant ASTM and ISO standards [26]. An image capture rate of 1 per second was deemed sufficient to capture enough data for strain analysis.3. The feed rate of 5mm/min was sufficient to allow relevant data to be recorded by the Aramis. The location of each dot is recorded in every image taken by the Aramis.2). Additionally.4.4 – Stochastic paint and mask area of sample. A more detailed description of the Aramis system and the benefits of using it for this thesis can be found in the appendix. Mask Area Figure 3.21]. Microscopic analysis of tested samples was used to measure the success of the 3 types of surface treatments (anodising. To separate the area of measurement from the background in the images. Figure 3. the success of each adhesive film in adhering to the surface of aluminium was studied. etching and no treating) against the results from the literature. Observations were made regarding glue area coverage and direction of shear tear (in reference to [9]). as seen in Figure 3.4) and tracks the movement of it during testing.18. the user defines a mask region. Surface topographies have been ranked in order of success and the surface treatment methods that produce these topographies have been tested (see Figure 2.process was employed using a white primer as the base.4 shows a painted sample with a sufficient pattern for detection and measurement by the Aramis camera apparatus.2 Microscopic Analysis Past work has extensively examined the microscopic surface topography of aluminium under various surface treatment methods [2. Similarities between surface topographies of the tested samples and the most favourable topographies from the literature were compared. The mask region covers the stochastic pattern and the area undergoing deformation. the mask region was a rectangle covering the bond area. 3. In the double lap shear experiments.2. Black aerosol paint was sprayed from a distance to create a stochastic pattern on the white base. The Aramis system locates each black dot (as seen in Figure 3. Such analysis helped to  . Changes in location of each dot from image to image create the strain map. understand why samples may have failed at the bond and why the glue may have not adhered to the surface of either the aluminium or Curv. . . with further investigation using a microscope to analyse the adhesive failure of these samples covered in section 4. Of the samples that were suitable for testing. With little effort and force.Chapter 4: Single Adhesive Layer Results The double-lap shear tests produced a range of different results. After the sample using Gluco adhesive film and untreated aluminium was removed from the gluing press it was observed that the bonding between the aluminium and Curv was unsuccessful. . depicted in Figure 4.1 Gluing Failure During the gluing process. A discussion of the results is conducted at the end of this section along with justification for further testing. 4. Additional samples failed during water jet cutting. 3 different types of failure were exhibited: adhesive. Firstly. During construction a number of the samples failed. Most of the untreated samples did not survive the cutting process. one sample failed after removal from the gluing press. 4.3.1.1 Untreated Specimens Preparation of the double-lap shear samples was conducted under the assumption that all surface treatment and adhesive film combinations would achieve bonding. Firstly. cohesive and structural. the weak bond between the aluminium and Curv was broken. Examination of the failed sample revealed that the Gluco adhesive film had bonded with the Curv polycarbonate. at 2 different stages. the footage captured by the Aramis was analysed to determine the shear and normal strain maps experienced by each sample up to initial failure. Samples were also viewed under a microscope to examine the adhesive film failure and surface topographies. Analysis of the results was conducted using 3 methods. Finally. the loading data collected by the Instron was analysed. Closer inspection revealed that the majority of the surface of the aluminium had failed to bond with the adhesive film. but exhibited poor adhesion to the untreated aluminium 5005. one of the samples failed to bond.1. Both of these results will be discussed in the following section. It was requested that cutting be ceased immediately and contact made if a sample were to fail. allowing the sample to fail. looking at the aluminium surface and the adhesive film and comparing the failed samples with the etched Gluco samples which successfully bonded. Another untreated aluminium and Gluco sample was constructed and glued. As discussed in the method.2 Cutting Failure After one of the untreated aluminium samples failed during preparation. Visual inspection of the new sample did not indicate any deficiencies in the bonding of the aluminium and Curv. 4.Figure 4.110 – failed during the cutting process. Further investigation will be covered in the microscopic analysis in section 4. As with the untreated samples.3.1 – A failed untreated aluminium specimen. It is possible that during the construction and gluing process one of the many variables present may not have been correctly controlled. an investigation will be discussed in section 4. Xiro melts at 165°C). failure must be caused by other factors. As the water jet cutting process does not heat Gluco above its melting point. More than half of the etched aluminium Gluco samples failed during the cutting process.1. The samples with an untreated aluminium surface – Gluco.601 and 23. warning was given to Serafin & Co. the water jet cutting process does not reach a high enough temperature to melt either of the adhesive films (Gluco melts at 155°C.3.  . Xiro 23. that additional samples might fail during the water jet cutting process. 3 additional samples failed. Failure therefore occurred either as a result of poor construction and gluing of all untreated samples or because bonding on an untreated aluminium surface is not possible [8]. During the cutting process. Given that the goal of the experiment was to look at the failure point of the adhesive. Common outcomes of the tensile testings were periods of extension where the loading application remained constant. regardless of the cause of failure.1 Failure Type The type of initial failure in each sample was determined by observing the images captured by the Aramis. the Instron extension data was used. but a detailed analysis is not relevant to understanding the bonding success and is therefore outside the scope of this thesis. not the ultimate failure of the double lap shear sample. These results can be largely attributed to processes other than de-bonding in the adhesive layer.2 Instron Double-Lap Shear Tests Many of the samples exhibited prolonged failure zones (example shown in Figures 4. loading graph of alkaline Xiro 23. A brief discussion and investigation will be developed in section 4. The type of failure that occurred initially in each sample is vital to understanding the success of the bond. de-lamination of the Curv). Adhesive failure: Failure occurs due to de-bonding of the adhesive from the surface of either the Curv or aluminium. Given the feed rate of 5mm/min.4.2. analysis of the Instron results was taken up to the point of initial failure. Cohesive failure: Failure within the adhesive film.110 sample 1.110 Sample 1 4500 4000 3500 Load (kN) 3000 2500 2000 1500 1000 500 0 0 1 2 2 3 4 5 6 7 8 8 9 10 11 12 Extension (mm) Figure 4. 4.2) before complete failure. Initial failure was defined as the first iteration (the INTSRON takes readings every 10 milliseconds) taken where the loading dropped by more than 1N. To find the time of initial failure. Etched Aluminium Xiro 23.2 – Extension vs.2.5 (for the most part. the extension of each sample at initial failure could be converted to a  . A brief description of the 3 possible failure methods is provided below: • • • Structural failure: Failure occurs within the Curv laminate. as shown in Figure 4. 601 Failure Type Adhesive Cohesive Adhesive and Cohesive Structural Cohesive Cohesive Figure 4. Adhesive failure in the etched aluminium Gluco samples suggests surface treatment was the cause of initial failure. Cohesive failure in both Xiro adhesive films indicates that initial failure was caused by weakness of the adhesive films. anodised aluminium with a Gluco adhesive film appears to have produced the strongest bond. The corresponding Gluco and Xiro 23. Figure 4. From these initial observations. Types of failure were observed to be uniform amongst samples of the same adhesive/surface treatment.3 – Failure methods for each sample set. Stress is given by the relationship between force and area.3. both etched) failed adhesively. Gluco has produced the most promising results. Shear stress is calculated by taking the tensile load from the Instron data and dividing it by the area of the bonds in the sample.2. The images were analysed to determine which of the 3 types of failure were experienced by each sample. as mentioned before. adhesive failure. Similarly.601 anodised samples failed cohesively. This hypothesis will be compared to the quantitative data analysis from the Instron and Aramis measurements. it can be concluded that the anodising process created a more successful surface topography for bonding than the etching process.110 Xiro 23.110 Xiro 23. At this stage of analysis. Figure 4.110 failed cohesively with both surface treatments and Xiro 23. Xiro 23. Firstly. 4. suggests the weakest point in the bond zone was the mechanical interlocking region between surface and adhesive. Two sample groups (Gluco and Xiro 23.4 groups the samples into their respective surface treatment/adhesive film combinations to compare the stress of each sample set. as shown in the formula below: Shear stress = F ÷ A Each double lap-shear sample had two bond regions with areas of 25mm x 75mm. structural failure suggests delamination of the Curv was responsible for initial failure. Surface Treatment Etched Adhesive Film Gluco Xiro 23.601.time and used to find the corresponding set of images taken by the Aramis. the Instron data was collected and analysed to measure the stress at failure of each sample. Of the three adhesive films. in the anodised aluminium Gluco samples.2 Stress at Initial Failure Having defined the initial failure point and the type of initial failure. .601 experienced both adhesive and cohesive failure. A few observations can be made from the findings presented in Figure 4.3 lists all the sample groups and the types of failure each underwent initially. in agreement with the literature.601 Gluco Anodised Xiro 23. 2 1 Stress (MPa) 0.601 is the weakest adhesive film.601 Etched Xiro 23. with the 3rd and 4th samples failing at stresses approximately 4 times smaller than the 1st and 2nd samples.8 0.110.5 contains the results.110 Etched Xiro 23.4 – Stress at initial failure of every tested sample.5) will aim to separate the two.Initial Failure Stress 1. The average stress of each set of samples was taken.4 Etched Gluco 1. after which the strength of the bond produced by the glue is compromised (The Xiro 23. along with a 95% confidence interval in an attempt to determine with statistical certainty which surface/adhesive combination produced the strongest bond. Formulas used in the statistical analysis can be found in the appendix.601 was a few years past its recommended shelf life). Gluco and Xiro 23.110 and Gluco samples.110 Anodised Xiro 23.601 samples produced unreliable bonds. . Data for the other two adhesive films. Xiro 23.4 0. Statistical analysis (Figure 4. Furthermore.2 0 1 2 3 4 Anodised Xiro 23.6 0. failing at lower stresses than all Xiro 23. the etched aluminium Xiro 23. with the mean of each sample set given along with the error bars indicating a 95% confidence interval. Figure 4. These results support the hypothesis that adhesive films have a maximum shelf life. was difficult to distinguish between.601 Anodised Gluco Sample Number Figure 4. 1 Shear Stress MPa S. The literature provided information on relevant features of the surface and adhesive film that would highlight the success of a bond.4 1.4 0. but has not separated the success of Gluco and Xiro 23.Average Stress at Initial Failure 1. The mean stresses for these adhesive films are also very similar.1 0 -0.9 0.3 Surface Topography An investigation of the surface of each tested specimen would provide further information in the success of the bond of each sample.2 0. . Success of adhesion between glue and Curv. With 95% certainty.601 is the weakest adhesive film. The 4 factors analysed in each sample under the microscope were: • • • Surface roughness and porosity. 4. there is no conclusive evidence to suggest which adhesive film withstands the highest stress before failure. a bond produced with Xiro 23.3 1.2 1. The Instron data has proven that Xiro 23.7 0.6 0.3 0. Comparing the statistical analysis to raw data results in Figure 4.1 1 0.3Mpa.601 is an unreliable choice of adhesive film. Success of adhesion between glue and aluminium.4.8 0. The confidence intervals of Gluco and Xiro 23. Mean Upper CI Lower CI Adhesive and Surface Treatment Figure 4.601 fails at the weakest average stress of the three adhesive films with both etched and anodised surface treatments. Xiro 23.5 – Average stress at initial failure of each sample type with confidence intervals.110 adhesive films.5 0.601 will fail at stresses between 0MPa and 1.110 for both etched and anodised samples overlap. The statistical analysis proved that Xiro 23. 50x in right image. Features compared were roughness depth and porosity.3.8 show a clear difference between the surfaces of untreated aluminium 5005.1 Surface Conditions of Aluminium and Curv Firstly. . 5x resolution in left image. indicating that the anodised aluminium has deeper holes than the etched aluminium. (refer to Figure 2. This is in agreement with the literature. straight parallel lines. flat surfaces [2].8 is both in and out of focus. 5x resolution in left image. Roughness depth and porosity directly relate to the success of mechanical interlocking between an adhesive and the surface of aluminium or Curv. The image taken on the right in Figure 4. Untreated aluminium is smooth with fine abrasive damage in thin. Etched aluminium has small holes. anodised aluminium has more frequent and larger holes than the etched aluminium.Untreated aluminium. Surfaces with varying depths and frequent holes adhere more successfully than smooth.7 . Figures 4. Figure 4.6 . This was achieved by comparing the surface treatments to those found in the literature. 4. anodised aluminium and etched aluminium.• Failure method of bond. 50x in right image.6) and indicates that anodised aluminium should promote the strongest bonds [2]. the 3 types of treated aluminium surface and the surface of Curv were studied. Figure 4.Etched aluminium.6 – 4. adhesive films have completely bonded to the Curv and failed to bond to the aluminium. Figure 4. Patches of Gluco were easily removed by hand. No bonding occurred between untreated aluminium and both the Xiro adhesives.9 shows the untreated aluminium and Gluco sample. the success of the 3 different adhesive films was investigated.8 .10 shows a Curv layer from a failed untreated aluminium sample. Small areas of Gluco have bonded to the aluminium.9 – Untreated aluminium Gluco sample. 5x resolution in left image. There are two . A closer investigation of the surface revealed why bonding failed. As outlined in the method. Figure 4. All adhesive types – Gluco.110 and 23.Figure 4. Xiro 23. Having established the superior surface treatment method to promote adhesion.601 – exhibited little to no adhesion between surface and adhesive. Figure 4. the untreated aluminium samples all failed before testing could be conducted. In these samples. but the Gluco layer had failed to adhere strongly to the surface. 50x in right image. This piece of Curv is representative of all Curv from untreated aluminium samples.Anodised aluminium. 110 samples underwent clear cohesive failure.12 shows the etched Xiro 23. preventing the adhesive film from bonding to the untreated aluminium.10 – Left image is 50x resolution of Curv surface.110. Curv is a polypropylene laminate and has a melting temperature of 160°C [19]. Firstly. Both the anodised and etched Xiro 23. One of the etched samples exhibited some adhesive failure.2 Xiro 23.  . Figure 4.3. it is likely that an untreated aluminium surface is smooth and does not facilitate bonding. The plain Curv has deep abrasions.10 compares a Curv surface without glue (left image) to one has bonded with Gluco (right image). It is therefore possible that some melting occurred on the outer surface of the Curv that touches the adhesive film. Figure 4. Figure 4. This may have caused the adhesive and Curv to mix in their melted states.110 samples was caused by internal stress in the adhesive layer. Right image is 50x resolution of Curv surface with Gluco. discussed in further detail in section 4. This theory was part of the motivation for further testing.5. The tears in the adhesive run parallel to the direction of force application (similar to the sample in Figure 4. Figure 4. suggesting a change in the surface of Curv that has been heated in the construction process. The second explanation is based on the melting temperature of Curv and the adhesive films. All samples were heated to a temperature of 155-160°C in the platen press. Note the difference in surface topography between the two pictures.110 Having established that untreated aluminium samples unsuccessfully bonded.11) suggesting that significant plastic deformation occurred in the adhesive polymer during failure.explanations as to why the untreated aluminium did not bond. It can be concluded that failure in most Xiro 23.110 sample under the microscope. whilst the Curv with Gluco is far smoother. 4.11 shows the cohesive failure of the anodised Xiro 23. comparisons of etched and anodised samples were made for each adhesive film.110 sample. The first adhesive film analysed under the microscope was Xiro 23. 11 – Anodised Xiro 23. 4.601 produced weak and unpredictable bonds. a load far .3 Xiro 23. Etched aluminium samples 3 and 4 failed at loadings approximately 25% of samples 1 and 2 (approximately 1kN compared to 4kN+).601 Thorough analysis was a necessity to determine why the samples with Xiro 23. initial cohesive failure highlighted. Cohesive Failure Figure 4.12 – Etched Xiro 23.Figure 4.110 sample.110 sample at 50x resolution.3. . 601 sample 4 exhibited successful adhesion between glue and aluminium surface. The adhesive sheared parallel to the direction of force application. The results suggest that mechanical interlocking between adhesive film and the surface of the etched aluminium was poor.smaller than all other samples (both etched and anodised).5kN). 2 and 3 underwent both cohesive and adhesive failure. suggesting the quality of Xiro 23. with ultimate failure occurring in an adhesive manner. As can be seen in Figures 4. Both of the samples that survived cutting and were used in lap shear tests failed adhesively.601 has declined significantly after passing its recommended shelf life. adhesion has failed over the majority of the aluminium surface.13.601 sample 3 resolution 50x. More than half of the samples failed during water jet cutting. patchy sections of adhesive covering little of the surface. with differing percentages of glue coverage on the aluminium surface.5-4. which failed at similar loadings (3. Samples 1. Other anodised/Xiro 23.601 samples experienced differing failure methods in the bond.601 to the aluminium surface. similar to that shown in . Etched samples 1 and 2 have experienced both adhesive and cohesive failure. Results were highly inconsistent amongst common sample types.13 – Xiro 23. Microscopic analysis showed patchy glue coverage. Anodised Xiro 23. white patches are adhesive film. The first notable difference between etched aluminium samples 3 and 4 and the others was the success of the adhesion of Xiro 23. Figure 4. 4.4 Gluco Etched Gluco samples were inconsistent.3. Initially. the glue has failed cohesively. with sparse.601 samples agreed with the Instron data. The microscopic analysis of Xiro 23. Figure 4.13. Inconsistent results amongst common sample types suggest that etching aluminium is not reliable at creating a strong bond. All of the anodised aluminium Gluco samples failed structurally, preventing microscopic analysis of the adhesive film and surfaces. These samples successfully withstood stress at the interface between adhesive film and surface and within the adhesive film layer. Initial failure was a result of de-lamination of the Curv. 4.4 Aramis Strain Analysis Whilst the Instron measured the loading in each sample, and allowed the shear stress in the bond to be calculated, it did not accurately measure strain. The Aramis system recorded all movements and distortions in the sample, and used every deviation to calculate a 2D map of the strain for each image captured. The 2D strain maps highlight the failure points of a sample and can help to explain why some surface treatment and adhesive film combinations are more successful than others. The change in strain from the start of a test to initial failure highlights the weak points in a sample. Ideally, a perfect sample has strain distributed evenly across the width and length of the sample. This indicates that all materials within the sample are undergoing strain at the same rate, implying that the load is evenly distributed. A sample exhibiting these strain patterns will fail with a clean break perpendicular to the direction of force application. Given the number of variables in the production process, the different properties of each material within the composite and quality of these materials, perfect strain distribution is impossible to achieve. Initial results from the Instron indicate that a perfect bond was not achieved in any of the samples. It is therefore relevant to look for a strain pattern that is both realistic and indicative of a successful bond. There are a number of ways in which strain can measure the success of a bond. Strain is proportional to stress. Regions of high strain indicate sections within the sample that are experiencing high stress and are likely to fail first. A poorly bonded sample will have the highest strain over the bonds. Given that aluminium has a far greater tensile strength than Curv (>100MPa for aluminium [20], 12MPa for Curv [19]), in a well-bonded sample failure will occur in the Curv laminate far earlier than the aluminium. The Instron data concluded that both anodised and etched Xiro 23.110 and Gluco samples failed at similar loads. With a 95% confidence interval, there was not enough deviation to prove with statistical significance the superior sample subset. Analysis of the 2D shear and normal strain measurements taken by the Aramis aided in ranking the adhesive film and surface treatments. The first set of results looked at the samples that were statistically proven to have the weakest bond strength, samples with the Xiro 23.601 adhesive film. Samples 3 and 4 of the etched aluminium group failed at far smaller loadings than all other samples, as shown in Figure 4.4.  An analysis of the strain concentrations and failure points of these samples can be used as a benchmark to measure the success of other samples against. Clear shear strain concentrations can be seen along the length of the two bonds in Figure 4.14. Strain is low in both the Curv and aluminium. Etched aluminium samples with Xiro 23.601 have produced a poor bond; the initial application of loading has strained the full length of each bond leading to quick failure. Figure 4.14 – Aramis normal strain capture of an etched Xiro 23.601 sample. The other Xiro 23.601 exhibited more evenly distributed strain maps. Sample 1 of the etched aluminium set exhibited similar shear strain distributions to those of Figure 4.14. Sample 2 of the etched aluminium set showed more promising results with shear strain more evenly distributed through the Curv and bond regions. The anodised samples exhibited more even shear and normal strain distributions than the etched samples, supporting the analysis of the Instron tensile loading results. In these samples, strain was concentrated at the top of the bonds, similar to Figure 4.15. One of these samples experienced strain in only one bond, one de-bonded before initial failure and two of them experienced strain distributions like those in Figure 4.15. Similar to the conclusions drawn from sections 4.3 and 4.2, the normal and shear strains on the Xiro 23.601 samples were unpredictable.  All Xiro 23.110 and Gluco samples failed at similar tensile loads. Xiro 23.110 samples all failed cohesively, where as Gluco samples failed both adhesively and structurally, as discussed in section 4.3. It can therefore be stated that these two adhesive films did not create equally sound bonds. The Aramis footage shows that the etched Xiro 23.110 samples failed in a similar manner. Three of the samples produced high strain regions along the bond zones, with lower normal strain across the width of the sample than the Xiro 23.601 samples, as seen in Figure 4.15. Additionally, three of the samples experienced cohesive de-bonding before initial failure. The other sample had strain concentrated at the top of the bond zones extending into the Curv layer. Figure 4.15 – Aramis normal strain capture of an etched Xiro 23.110 sample. The four anodised Xiro 23.110 samples failed cohesively like the etched samples, but the measurements by the Aramis indicate different strain distributions. Shear and normal strain in all four samples was concentrated at the top of the bond zone, with little extension along the bond or into the Curv and aluminium layers. Maximum normal strain followed both bonds during cohesive failure, as shown in Figure 4.16. Furthermore, all samples failed cohesively as the tensile load was still increasing. De-lamination within the Curv layer was responsible for failure after the initial failure point. The etched Gluco samples were inconsistent. the Aramis footage could not accurately pinpoint the type of initial failure. In two of the samples. Three of the samples failed during the water jet cutting stage. The second sample had strain concentrated at the top of each bond zone until initial adhesive failure. see Figure 4. The two samples that survived cutting failed adhesively during tensile loading. The anodised Gluco samples exhibited the most even normal and shear strain distributions of the three adhesive films.16 – Aramis shear strain capture of an anodised Xiro 23.17.14. All of the samples had strain concentrated in the Curv layer from application of loading until initial failure. In these samples cohesive and structural failure occurred within a second of one another. it was difficult to determine whether initial failure was cohesive or structural. Set at a frame rate of one image per second.Figure 4. .110 sample. The first sample displayed clear strain concentrations along both bonds. Three of the samples experienced initial failure at two points in the Curv close to the bonds. The strain patterns measured by the Aramis suggest that the adhesive bond produced between an etched aluminium surface and Gluco adhesive film is similar to those in Figure 4. All four of the samples failed structurally through de-lamination of the Curv. The gluing process applies heat and pressure to the sample to create the bond. The Aramis analysis suggests that anodised aluminium with Gluco adhesive film created the strongest bond. the minimum melting point of Curv is 160 °C [19].Figure 4. Taken from the manufacturer’s data. it was clear that anodised aluminium Gluco samples produced the strongest bond. Maximum normal and shear strain was localised in the Curv layer in all of the samples. 4. Strain distributions of the Xiro 23. All other combinations failed either cohesively or adhesively.17. strain was concentrated on both the bond regions and the outer layer of the Curv.110 and Gluco samples could not separate the two groups. complete failure occurred in the outer laminate layers of the Curv. close to the bonding zones. In other sample types. as seen in Figure 4.17 – Aramis normal strain capture of an anodised Gluco sample. The temperature of the press is between 155-160 °C. The outer layers of the Curv laminate may be compromised during gluing of samples. During the gluing process. The statistical analysis of the Instron tensile loading of the Xiro 23. From the analysis. The microscopic analysis agreed with the literature that anodised aluminium creates the most favourable surface topography for mechanical interlocking to occur.5 Scope for Further Testing From the first round of testing a few conclusions can be drawn. Strain maps captured by the Aramis assisted in distinguishing between the success of each surface treatment and adhesive film. the adhesive film .601 samples proved that the adhesive film was unreliable and produced bonds of inconsistent strength when used beyond the recommended shelf life.601 samples were inconsistent. Analysis of all Xiro 23. some with more favourable strain concentrations than others. In these samples. 110 and other parties working with the adhesive film based in England strongly suggested that bonding is achievable without prior treatment of the surface. obstructing it from bonding to the surface of the aluminium. These partially melted laminates layers may have weaker material properties than the rest of the Curv. Referring to Figure 4. Firstly. the equivalent anodised aluminium samples did not. Instron and Aramis analysis.601 proven in the microscopic. Five samples were made for each test combination.melts to fill the pores on the surface of the aluminium. a total of 8 different sample types were to be tested. it was removed from the second round of testing. given the unreliability of Xiro 23. Etched aluminium Gluco and Xiro 23.110 6 5 3 5 Gluco 6 5 3 5 Xiro 23.18 – Parameters for tests of the adhesive film thickness. Increasing the thickness of the adhesive layer may allow the glue to both bond with the surface of the aluminium and fuse with the melted out layers of the Curv laminate. due to poorer results than the anodising treatment.3 lists the type of failure for each set of samples. Surface Treatment Anodised Number of Layers Number Of Samples 3 5 Gluco 6 5 3 5 Xiro 23.18.601 samples failed adhesively. Adhesive Film Untreated . explaining why failure is often observed here first. Figure 4. It is possible that the outer layers of the Curv may experience partial melting. The next series of tests included anodised and untreated aluminium samples. Both Gluco and Xiro 23.3. given that the temperature approaches Curv’s melting point. 3 and 6 layers of adhesive film were tested.110 6 5 Figure 4. The adhesive film may fuse with the Curv. Given these results and hypotheses. with specific reference to Figure 4. The etched surface treatment was also removed. In the second set of testing. Further consultation with the manufacturer’s of Xiro 23. A few changes were made to take into account the successes and failures of the initial tests. a second round of testing was proposed.110 adhesive films were tested again. Preliminary results suggested that increasing the thickness of the adhesive film did not aid in creating a bond on the surface of untreated aluminium. 16 failed during water jet cutting. Further investigation of the untreated samples will be covered in later sections. • To measure the viability of bonding on untreated aluminium surfaces. they tend to rotate slightly off axis when the final few turns of the vice are applied. The success of anodised Gluco samples in initial tests suggested bonding with this surface treatment and adhesive film was most favourable. Initial observation suggested that the bond was of sufficient strength and that additional layers of adhesive film had allowed adhesion to occur on an untreated surface. to measure the strength of the bonds and compare to bonds formed with an anodised aluminium surface. This torque force was large enough to tear the bond in all of the untreated 3 layer Gluco samples.110 adhesive film (given the manufacturer’s claim that bonding on untreated aluminium was possible).110 sample survived and was tested under tensile loading. All but one of the anodised Gluco samples with 6 layers of adhesive film failed during cutting.110 adhesive film. The goal of the secondary tests was to answer some questions brought up from initial tests and to distinguish between similar results. Namely.110 . and the optimal thickness of the bond. The samples that survived the water jet cutting stage were prepared for testing in the Instron. The untreated 3 layer Xiro 23. When tightening the grips and locking the sample in place. Of the 20 samples produced with an untreated aluminium surface. specifically with the Xiro 23. as did one sample with 3 layers of Xiro 23. To find which adhesive film . All of the samples with 6 layers of adhesive film failed. This rotation is caused by the grips that can rotate a few degrees either side of their main axis due to the mechanism of their insertion and attachment.Gluco or Xiro 23. • • 5. If bonding is possible with untreated surfaces.produces a stronger bond.1 Untreated Samples All of the untreated aluminium samples successfully bonded during the gluing press. Inspection of the failed samples revealed . Loading a sample in the Instron required application of a small torque force about the major axis of the sample. Four samples with 3 layers of Gluco survived. although it has been proven that bonding between the tested adhesive films and untreated aluminium 5005 is not possible.Chapter 5: Multiple Adhesive Layer Results Further testing was conducted using multiple layers of adhesive film. 1 sec). Figure 5. defined by a loading drop of more than 1N. as with the single adhesive layer tests. where the other is flat.1 shows one of the failed samples.2.  .1 – Anodised 6 adhesive layer Gluco sample.the cause of the failure. In the multiple adhesive layer tests.2 Instron Double-Lap Shear Tests 5. Unfortunately. This difference in surface shape has prevented the Curv from bonding to both sheets of aluminium.2. all double-lap shear samples were loaded until a 30% drop in loading occurred over a single iteration (0. 5. Reusing the same test method ensured that consistency was maintained. One of the aluminium layers has a slight bend. The type of failure for each sample at this point was recorded using the Aramis footage and is listed in Figure 5. Analysis of the Instron data was taken up to the initial failure point. allowing both sets of results to be accurately compared. Figure 5. time constraints prevented another sample from being produced.1 Failure Type The testing method used in the single adhesive layer tests was reapplied in the multiple adhesive layer tests. 800 0. Samples in the second round of testing were of the same dimensions as those in the first.200 Stress (MPa) 1. The Instron was set with a 100kN loading cell and an extension rate of 5mm/min. The Xiro 23.2 Stress at Initial Failure Instron tensile loading data was converted to stress.700 1 2 3 Sample Number 4 5 Anodised Xiro 3 Anodised Xiro 6 Anodised Gluco 3 Anodised Gluco 6 Untreated Xiro 3 Figure 5.3 groups the samples into their respective surface treatment/adhesive film combinations to compare the stress of each sample set.110 6 During Cutting Figure 5. 25mm x 200mm. 5. The anodised Gluco samples also experienced structural failure. Adhesive Film Initial observations suggested that favourable outcomes had occurred.100 1.3 – Stress at initial failure grouped into sample types. Stress at Initial Failure 1.2 – Initial failure types for each sample set.110 has improved the strength of the bond.900 0.Surface Treatment Anodised Untreated Number of Layers Type of Failure 3 Structural Gluco 6 Structural 3 Structural Xiro 23.300 1. Figure 5. Analysis of the Instron and Aramis data was undertaken to determine whether bond strength was improved.400 1. All anodised samples experienced initial failure structurally through de-lamination of the Curv laminate. .000 0.110 6 Structural 3 During Cutting Gluco 6 During Cutting 3 Cohesive + Adhesive Xiro 23. samples with thicker adhesive films failed structurally.110 adhesive film failed cohesively on an anodised aluminium surface in the single layer tests. Thickening the adhesive layer of Xiro 23. both in the single and multiple layer tests.2. . statistical analysis of the data was undertaken to attempt to aid in differentiating between all sample types. creates a stronger bond than Gluco on an anodised surface. It is difficult to determine the difference between 3 and 6 layers of adhesive film from the results in Figure 5.00 0.5. With a single layer of adhesive film. Similarly to the single layer samples. we can determine the number of layers of adhesive film required to create the strongest bond.110 samples produced stronger bonds than those with Gluco.10 1. Note that the anodised Gluco test with 6 layers and the untreated Xiro 23. Average Stress at Initial Failure (Multiple Adhesive Layers) 1. Xiro 23.4 includes the results. Again.4. again on an anodised surface. At this point. mean. Mean Upper CI Number of Layers and Surface Treatment Figure 5. Figure 5. specifically the Xiro 23. for both Gluco and Xiro 23. With a thicker adhesive layer.80 Lower CI S. with a thicker layer of adhesive film than provided by the manufacturer. These two results harbour no statistical significance and are included for simple comparison.05 1. 2 or 3 layers.20 Stress (MPa) 1.Thickening of the adhesive layer produced stronger bonds.30 1.25 1. Two conclusions can be drawn from the statistical analysis in Figure 5.110 samples with 6 layers of adhesive film did not create stronger bonds than those with 3 layers. percentage error and confidence intervals were calculated.85 0.15 1.110 test with 3 layers do not have confidence intervals as only one sample of each type made it to the testing stage. Therefore the optimum thickness of an adhesive layer is 1. Gluco creates the strongest bond.4 with Figure 4.90 0. Xiro 23. Both the Gluco and Xiro 23.3.110 samples.95 0. it has been established that an anodised aluminium surface is superior to .110. By comparing the data in Figure 5. standard deviation.4 – Average stress at initial failure of each sample type with confidence intervals.110. with the mean and 95% confidence interval for each sample type shown (Formulas used in the statistical analysis are found in the appendix). which outlined what a perfect strain distribution looks like and realistic strain distributions to be expected in well-bonded samples. The Instron load measurements for these samples were similar to one another. Adhesive Gluco Number of Lower C.843 0. Whilst the mean tensile stress increased from a single adhesive layer to 6 adhesive layers.084 1.920 1. Analysis of the strain distributions up to the initial failure point of each sample helped to rank them.183 Structural 3 layers 0.110 1 layer 1. Although the statistical analysis of the Instron data did not differentiate between the different adhesive thicknesses.5 – Comparison of confidence intervals of all Xiro 23. The range of 95% confidence for the 3 adhesive layer tests lies below the range of 95% confidence for the single adhesive layer tests. All images used in this section showed either normal or shear strain maps measured by the Aramis. All of the anodised samples failed initially within the Curv layer. 3 and 6 adhesive layers of Xiro 23. Failure 1 layer 1.110 samples did not conclusively increase the strength of the bond. but all the 3 and 6 layer samples failed structurally within the Curv layer.  .140 1.003 Structural 6 layers 0.247 Cohesive 3 layers 1.028 1.I. Normal strain can help to indicate the delamination of the Curv (see Figure 5. the 95% confidence range of the single adhesive layer tests was wide enough to include the range of both multiple adhesive layer tests.138 1. The results of the Aramis strain footage were therefore vital to differentiating between the successes of the anodised aluminium multiple adhesive layer samples. the observational data did. Figure 5.an etched surface and an untreated surface. Therefore.180 1.106 1. the optimal thickness for an anodised Gluco samples is either one or two layers of adhesive film.269 Structural Figure 5. Mean Upper C. Increasing the adhesive layer thickness of anodised Gluco samples did not improve the strength of the bond.110 samples. Shear strain highlights the regions of high stress within a sample.110 and Gluco tests.176 Structural 6 layers 1. 5. Increasing the thickness of the adhesive layer in Xiro 23.100 1.91 Structural Xiro 23.4. the addition of further layers reduces the bond strength. Further explanation of strain distributions and expected results was covered in section 4.110 samples failed cohesively. Single layer Xiro 23.110 created the strongest bond.5 compares the statistical data for all anodised Gluco and Xiro 23.028 1.6).I.3 Aramis Strain Analysis An explanation of the Aramis testing procedure is outlined in the Method section. 6 – Normal strain of an anodised 6 adhesive layer Xiro 23. Samples with multiple adhesive layers showed promising results from the Aramis.Figure 5. Analysis of the strain map on the untreated aluminium Xiro 23. All samples had maximum shear and normal strain distributions in the Curv layer at the top of the lap bond regions. most of the anodised Gluco samples with 6 layers of adhesive film failed during the water jet cutting process. As discussed earlier in this section.7 shows the normal strain in the sample and highlights the point of initial failure. This sample failed through de-lamination of the outer layers of the Curv. strain patterns of 3 and 6 adhesive layer samples were quite different and highlighted the differences in bonding strength between the two. Whilst it appears to be failure in the bond. supporting the theory that the Curv partially melts during gluing.110 sample at initial failure.110 sample revealed poor adhesion.  . Furthermore. strain within the mask area was minimal and evenly distributed across the three material layers. In these samples. Figure 5. One sample did survive and was tested in the Instron. it is the outer layers of the Curv that are delaminating. All untreated samples in both sets of experiments failed with no adhesion between the surface and any type of adhesive film. As stated. It is therefore likely that the surface was accidentally roughened by abrasion during the manufacturing process.Initial Failure Figure 5. 3 and 6 layers samples behaved differently under loading. as highlighted in Figure 5.110 and the aluminium surface. exhibited maximum normal and shear strain in the Curv laminate. promoting some form of mechanical interlocking between Xiro 23. All samples failed initially in the Curv laminate at the top of or above the bonds. . The 3 adhesive layer samples. The single untreated aluminium sample that survived until testing failed adhesively.8.7 – Normal strain of an anodised Gluco 6 adhesive layer sample. Inspection of the sample after testing revealed an area of cohesive failure in the centre of the lap joint.110. both Gluco and Xiro 23. Failure occurred in one of the bonds. with Aramis footage revealing normal strain concentrations extending down along the bond from the top of the adhesive fail point. 110 sample. . Higher normal strains are indicative of severer de-lamination of the Curv.8). The normal strain is dependent upon the severity of the de-lamination within the Curv. The mask zone had an even distribution of normal and shear strain (blue colouration if Figure 5.110 sample 2 experienced initial de-lamination of the Curv outside the mask zone. Normal strain is concentrated on the Curv layer extending above the bond region. the shear strain (Curv fails at 20% shear strain) at failure of the Curv laminate was in agreement with the literature [19]. Note that Gluco sample 2 and Xiro 23.9.Figure 5. Both shear and normal strains for each 3 adhesive layer sample are listed in Figure 5. indicative of an ideal bond.8 – Normal strain of an anodised 3 adhesive layers Xiro 23. Furthermore. The strain readings for these samples are somewhat inaccurate. The bond zones had evenly distributed normal and shear strain. As discussed earlier.110 adhesive film is 0.110 samples experienced similar strain to the 3 layer samples.10 – Samples with 6 layers of adhesive film. compared with 2mm thick aluminium sheets and 3mm thick Curv laminate.5% 4 21. The Aramis footage revealed differences between the number of adhesive layers. during construction the adhesive film melted and the pressure forced much of it outside above the bonds.9% 6.9 – Shear and normal strain for adhesive film thickness tests.2% 8.5% 3 20. from the Instron analysis.5% Xiro 23.1% 4 18. highlighted in Figure 5.Shear strain (ε Y) ε Normal Strain (ε X) ε 1 19.8% 5 21.4% 6.7% Figure 5.110 1 21. The accuracy of the shear and normal strain measurements taken by the Aramis was compromised by the structure of the composite. 6 layers of Xiro 23.6% 2 3 19. Adhesive Film Gluco Sample Number The 6 adhesive layer Xiro 23. All samples failed within the outer layers of the Curv laminate.5% 8.10.1% 5. strain concentrations were at the top or above the mask zone. The thickness of the adhesive film may be too great. specifically the aluminium. the 6 adhesive layer Gluco samples experienced problems with poor material quality.8% 6.0% 12. . the tensile strength of 6 and 3 adhesive layer Xiro 23.3% 14.2% 2 10. Figure 5.8% 7.3% 5 19. Statistically.110 samples were very similar.3mm thick. thickening the adhesive layer does not promote adhesion between untreated aluminium and Curv. where maximum shear strain is experienced on the excess glue when it separates from the top of the aluminium. All other samples. However. suggesting that the tensile strength exhibited by this sample during testing was due to accidental roughening of the surface before the sample was glued. the glue. During tensile loading and extension the excess adhesive film separated from the top of the aluminium layers. The large green area of shear strain indicates that the Curv laminate is experiencing greater tensile stress than the bonds and the aluminium.11 – Shear strain capture of a 6 adhesive layer sample. Only one of the 40 untreated aluminium samples survived until testing. 5.4 Discussion Increasing the thickness of the adhesive film definitively increased the tensile strength of the bond between aluminium and Curv. experienced the largest normal and shear strains.110 or Gluco. Untreated aluminium 5005 will not bond with Xiro 23.11 shows the strain readings of a 6 adhesive layer sample.The build up of adhesive film above the bonds was enough to be captured by the Aramis camera. . similar to those in the first round of testing. Additionally. Figure 5. Figure 5. having weaker mechanical properties than both the Curv and Aluminium. failed during the water jet cutting process. Two theories arose from the results of the multi-layer anodised sample tests.12 – Multiple adhesive layer sample after testing in the Instron.110 and Gluco adhesive films produced a greater tensile modulus than recorded by the Instron. As the purpose of testing was to create a composite where tensile failure occurs in the Curv laminate before the adhesive bond. All samples failed in the outer layers. Firstly.12. . all anodised samples failed initially within the Curv layer. The second round of tests reinforced the second theory that the outer layers of the Curv laminate experience partial melting when in the gluing press. Figure 5. as shown in Figure 5. suggesting that the bond created by both Xiro 23. those that were left under loading until complete failure exhibited these failure patterns along the full length of the mask zone. the actual tensile strength of the bond is not necessary. Three variables were measured: the surface treatment. confirmed that with thicker adhesive layers. The first round of tests proved that anodising aluminium 5005 surfaces created the most favourable surface topographies for adhesion. Of the adhesive films tested. In the automotive industry.2 Further Work FMLs have seen application in the commercial sector via the aerospace industry.1 Conclusions This thesis investigated various methods of bonding Curv-aluminium FML. and untreated surfaces would not bond at all. Glass and aramid fibre based FMLs have been used in fuselage.601 produced unreliable bonds. it was found that there is an optimal thickness for each adhesive film with thicker or thinner adhesive layers having weaker bond strengths. Two sets of tests were conducted that determined the optimal set of conditions to create the strongest bond. where the thickness of the adhesive film was increased. Introduction of the work done in this thesis into future research involving aluminium-polypropylene FMLs could measure the changes to other properties of the composite. Xiro 23. wing and aircraft floor construction. The second round of tests.110 created the strongest bond and were equal in strength to 1 or 2 layers of Gluco adhesive film.110 created stronger bonds than Gluco. further work would implement the results in current testing and production. Failure in the Curv laminate in the anodised Gluco samples suggested that this combination produced the strongest bond. 6. the adhesive film. However. Xiro 23. aluminium-Curv FMLs aim to achieve similar material properties at a cheaper production cost for usage as panelling. Etched surfaces were not as reliable as anodised surfaces.Chapter 6: Conclusions & Further Work 6. and the thickness of the adhesive film.601.  .110 were statistically proven to produce stronger bonds than Xiro 23. with wide variance in the results. Gluco and Xiro 23. 3 layers of Xiro 23. Anodised samples made with these adhesive films and specified thicknesses failed through delamination of the Curv. Having determined the ideal surface treatment and adhesive film for bonding. Given that the work conducted in this thesis created a bond with greater tensile strength than the polypropylene laminate. Additionally. . testing of the change in impact properties would be an obvious extension of the work. peel testing using the strongest bond conditions found in this paper would further the understanding of aluminium-polypropylene bonding. . 205 + 1. A 95% confidence for the shear stress of a sample says that there is a 95% chance that a new sample tested will have a shear stress in the same range as the confidence interval.096 .154 MPa Mean = (1.138)2 + (1.138)2 + (1. The data used in the example is from all 5 Xiro 23.019 MPa 95% Confidence Interval = [1. An example of how a confidence interval was calculated is set out below.176 MPa The tables below contain all values calculated in finding the confidence intervals for the results.154 .116 MPa = Sample 5: 1.138 + (0.205 MPa = Sample 4: 1. the second table contains values relating to Figure 5.1.5.096 + 1.205 . .138)2 + (1.043/(√5) = 0.1.1.96)] = 1.110 anodised aluminium 3 adhesive layer samples: Shear Stress = Sample 1: 1.138)2}] = 0.096 MPa = Sample 2: 1.116 + 1.138 – (0.019*1.4.154)/5 = 1.118 + 1.118 MPa = Sample 3: 1.138)2 + (1.118 .116 .100 to 1.1. The first table contains values relating to Figure 4.96)] to [1.Appendix A1 Statistical Analysis The goal of the statistical analysis of the loading data was to determine the significance of the results.138 MPa Sample Standard Deviation = √[(1/4)*{(1.043 MPa Standard Error = 0.1.019*1. By capturing multiple images during mechanical testing. 3 layers Anodised Xiro 23.028 0.056 1.176 1. the Aramis records an evolution of strain from steady state until failure.672 0.843 Statistical data analysis used in Figure 5.247 1.  .Sample Type Anodised Gluco Anodised Xiro 23.835 1. In terms of the work in this thesis. 3 layers Sample Mean (MPa) Sample Standard Deviation Standard Error Lower Upper Confidence Confidence Interval Interval 1.4 in the table above.056 0.028 Statistical data analysis used in Figure 4.865 0.106 0. A2 Aramis Strain Evolution The Aramis system has the unique ability to create 2D and 3D strain maps from images captured by a camera setup.105 0.601 Etched Gluco Etched Xiro 23.973 0.118 0.601 Xiro Old Sample Sample Upper Lower Mean Standard Standard Confidence Confidence (MPa) Deviation Error Interval Interval 1.177 0.289 -0. Sample Type Anodised Xiro 23.063 0.5 in the table above.923 0.110.091 0.043 0.021 1.028 1.138 0.041 1.044 0.047 1.110 Anodised Xiro 23.083 0.630 0.998 1.019 1. 6 layers Anodised Gluco.176 1.336 1.003 0.059 0.084 0.475 0.183 1.079 0.112 0.110. the Aramis allows analysis of the development of normal and shear strain in double-lap shear samples.040 1.146 0.110 Etched 23.138 0.269 1.100 1.079 0. The first image is normal strain before testing.  .as the layers of Curv delaminate. Normal strain at this point is at a maximum at the top of the sample in what would be the Curv laminate.Below are two series of images taken from an anodised aluminium Gluco single adhesive layer sample. The first set of images captured from the Aramis looks at the development of normal strain. Images 2 and 3 were taken during tensile loading but before initial failure of the sample. These samples were observed to have ideal adhesive bonding. at this point the sample has begun to delaminate. the second set looks at the development of shear strain. Image 4 was taken at initial failure. Normal strain sharply increases – seen in image 5 .  . This was due to the direction of force application. shear strain was parallel to tensile loading. Images 2 to 4 show the development of strain in the sample. normal strain was perpendicular. Shear strain developed faster than normal strain. Image 5 shows the shear strain at initial failure. Shear strain indicates which layers in the FML were experiencing the greatest tensile stress.Image 1 depicts the shear strain before testing. in this case strain develops in the Curv laminate. Normal strain indicates where separation and delamination was likely to occur. The examples of shear and normal strain above highlight the power of the Aramis. 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